Corporate Liable Credit Cards

[Megalita]

What is the name of your state? Virginia

I worked for a small company almost a year ago which had a corporate liable Amex card for which I had one and each of the owners had one. I made many purchases for the company over several years on this card, but have not used it in at least a year and haven't worked for the company since June 1, 2005. Recently a collection agency contacted me because my former employer has not paid the card and Amex has turned the collection over to them. They are saying that I am responsible for any charge I signed off on even though it was for the company and I haven't even used the card in at least a year and no longer even work for that company. My former employer is most likely going to declare bankruptcy (he has always had problems with prompt payment which is one of the reasons I left the company). Can Amex come after me for the charges I signed off on even if they were legitimate business expenses? Also how do I prove that I am not responsible for them, do I even have to prove it?