Claim loss as business guarantor

you should discuss this situation with a lawyer. there are legal issues that could come up. is your payment a contribution to the capital of the business?  if so, then does this loss gets split among the partners in proportion to their profit and loss % as specified in a partnership agreement assuming there is one. this could leave you with part ordinary loss and part capital loss. another issue do you have an enforceable claim against any of the other partners for their share? such a claim could bar a tax deduction unless you pursue the others for their share. 

tax laws also may restrict your ability to deduct the "loss" as guarantor.  

In order to get an ordinary or capital loss, the taxpayer must also be able to demonstrate several facts. The obligation to make good on the guarantee must have been enforceable by the lender before the original debtor defaulted on the debt.

Also, the guarantor must have received some benefit as a result of making the guarantee.