Despite the promulgation of the "PSM Standard" by OSHA in 1992, chemical accidents continue to occur at an alarming rate. As part of the process safety information element of the standard, OSHA requires reactivity data on the chemicals in the process. One piece of this reactivity data is a chemical interaction matrix. Several software programs provide simple approaches to generating these interaction charts for common materials using assigned reactive groups. If your process involves materials not included, you are left to determine the reactivity and incompatibilities on your own. This paper will explain a process for assigning reactive groups using chemical classification and structural analysis. This process can be used for less commonly understood materials such as additives, lubricants, mixtures, products, etc. that may come into contact with process chemicals. (C) 2015 American Institute of Chemical Engineers