Ion-assisted OH measurements are presented from two different standpoints. First, instrument operation and calibration are discussed along with recent improvements in measurement techniques. A chemical model of inlet titration chemistry in both clean and polluted air is also included in this section. The results of OH measurements using the present technique are presented for the entire tropospheric OH photochemistry campaign. Hydroxyl radical concentrations of 2--4¿106 molecules cm-3 were typical at noon for clean, clear days. In polluted air masses, OH concentrations up to 8¿106 molecules cm-3 were observed. Variation of OH with changing and differing air masses is clearly shown and discussed.¿ 1997 American Geophysical Union