The HNu PI 101 photoionization detector (PID) based instrument has historically been used for field investigations and site safety determinations at Ecology and Environment, Inc. (E & E). Photovac, Inc. has more recently introduced the TIP II, which operates on the same principles and serves the same purpose as the HNu PI 101. Both instruments can provide nonqualitative, semiquantitative data regarding the presence of ionizable species in ambient air. E & E, under its Zone II Field Investigation Team (FIT) contract with the Environmental Protection Agency (EPA), has completed a comprehensive side-by-side evaluation of the TIP II and PI 101 to determine the utility of the TIP II for the EPA preremedial program. Manufacturer's literature was considered, including instrument descriptions, and design specifications, as well as a comparative evaluation of instrument user manuals. Laboratory test results of instrument operational parameters were evaluated, including battery tests, evaluation of controls, zero stability, flow rates, instrument response time, and response fluctuation. Instrument performance with selected standard gases was compared in a controlled environment. An examination of field instrument performance was conducted. Test results indicate that there are differences in the performance and operation of the PI 101 and TIP II. These differences should be considered by field personnel when the use of an air monitoring instruments are used for field operations.