Here is a list of the largest merchant acquirers in the U.S.  as estimated by the Nilson Report.  In this context the definition of Acquirer is a little bit strained, because it includes ISOs and processors at multiple levels of the value chain.  For example, EVO Merchant Services (#14) is known to be an ISO that sells processing provided by Global Payments Inc (#5).  I am not sure what methodology Nilson used to make these estimates, they are probably directionally correct, but there is a chance that some double-counting is involved.

One of the most confusing parts of credit card processing is that the merchant's daily batch statement that is either printed out from their terminal (in the case of a physical merchant) or emailed from their payment gateway such as Authorize.net (in the case of an online merchant) does not match the amount that is put into their bank account. 

I created this site because when I started my business two years ago, I had no idea how a merchant account (credit card processing) worked.  When I filed my incorporation papers with the state I got phone calls from a dozen aggressive salesmen selling merchant accounts.   

I knew that the costs associated with my merchant account would be a big part of my business expenses (more than 2.00% of sales) and I was nervous about making such a big decision without knowing all of the facts, but I couldn’t find any good, unbiased information to help me negotiate.