If tyres have been filled with nitrogen, can they be filled with air at another time?

Question from visitor, 08/01/2010 :If tyres have been filled with nitrogen, can they be filled with air later?
Answer by rezulteo, 08/01/2010 :

You can re-inflate your tyres with air even if they have been inflated with nitrogen on an earlier occasion but you should have the pressure checked more often.