The first ion-assisted OH measurement instrument was developed and field tested in 1989. Since that time both instrument and technique have evolved substantially and several potential measurement interferences have been investigated. Included among these are reactions that could compete with the H2O/SO3 reaction, the effect of H2O on the NO-3 ⋅ HNO3/H2SO4 reaction, and potential wall losses. These investigations have also provided data of relevance to atmospheric ion and neutral chemistry for the two reactions listed above, as well as for the OH measurement itself. Measurements made at Mauna Loa Observatory in Hawaii show for the first time a diurnal variation in tropospheric OH which spans more than 2 orders of magnitude. Nighttime measurements revealed hydroxyl radicals in the 104 molecules cm-3 concentration range well after dark. ¿ American Geophysical Union 1995