This product utilizes a bicarbonate buffering system. However, there are no anticipated problems with the addition of HEPES. We do offer RPMI formulations containing both sodium bicarbonate and HEPES, such as Products R5886 & R7638.
Please see the RPMI formulation tables at the link below: https://www.sigmaaldrich.com/deepweb/assets/sigmaaldrich/product/documents/203/777/r8758for.pdf