I spent last summer mainly cycling, but after losing some weight over winter I have been able to return to doing some running as well.

I am finding that even though I weigh less than last summer, when I am running my cycling performance is not so good. And if I do a good week or so only cycling, then my next run is hell.....

My question is does combining these two sports result in a performance degradation in either, and if so how significant?

Without knowing a lot of specifics, like how far you're running/riding, how fast, and what sort of shape you're in -- moderately fit or extremely fit -- it's hard to gauge. But running and cycling use different muscle groups. You're building aerobic and anaerobic capacity in both, and strength too, but the specificity in training for one does not transfer necessarily to the other sport. If you're on the fine edge of fitness, running will not keep you on that edge for cycling and cycling will not keep you etc. etc. etc.

If you're just generally cross training for fitness, specificity in one area may result in a loss of fine tuning for the other. But now you're getting into some arcane concepts in training, and without more data it's hard to know what you're experiencing.

I spent last summer mainly cycling, but after losing some weight over winter I have been able to return to doing some running as well.

I am finding that even though I weigh less than last summer, when I am running my cycling performance is not so good. And if I do a good week or so only cycling, then my next run is hell.....

My question is does combining these two sports result in a performance degradation in either, and if so how significant?

It would be better to guage with some more information.

For myself I try to squeeze all my runs onto days with out cycling except for what I call "brick Sunday." That leaves four rides, four runs per week with each week having either a hard run or a hard ride as well as an alternate longish run and long ride. (Plus a couple of swims per week). What becomes apparent is the need to build up to the amount of extra training without allowing fatigue or dullness take over.

If I am concentrating and building slowly then I normally can expect good gains and "decent" results at events. But it is all in what you train for, I could be a better runner if I cycled less added a fifth run a week and made sure that my longish run became a LONG run. And the same with cycling take away all of the running miles and put a 5th or 6th day on the bike and I am sure I could be faster...but...I have a tendancy to burn out that way. Cross training has always been a better way for me to stay focused and to key in a few tri's along the way until the season ender.

Quick advice...don't go a week without running, for people like myself who are not naturally gifted at the sport physically it would be like starting over.

level will be excellent with cross training. This is also great for injury prevention. I know plenty of cyclists who will run on rainy or rest days from the bike. I usually run 3-4 miles 5 days a week and try to cycle about 10-15 miles 3 days a week with longer rides on the weekends. Personally I think it's a great combination.

My question is does combining these two sports result in a performance degradation in either, and if so how significant?

I spent part of last season running 5-6 days a week and riding 7 days a week. I got faster with my running, but was often too fatigued to do well in bike races. I quit running and started kicking butt on the bike.

In my experience, cycling will help your running, but running will NOT help your cycling. You'll be faster and stronger on the bike if you stick to just cycling.

YMMV, of course. ;)