Yes, you can use the steripen with a sigg. There was an issue that it was hard to "stir" properly since the sigg opening is too small. However this is a new sigg/steripen accessory it help with this. Its a small rubbery ring that goes over the mouth of the sigg. You insert the steripen and the ring causes a seal. Then you can turn the sigg upside down and agitate so that all the water gets treated properly. The ring is less than $3. Only possible issue you may have is that you can't see through a sigg to make sure the light stayed on. However, the steripen does have an indicator light, so this shouldn't be too much of an issue. As an added bonus, the rubbery ring is nice and soft and isn't as cold when drinking out of the sigg bottle in winter.--------------------That does indeed solve the problem. The stirring is to make sure it's all contacted sufficiently, but that would do it.