Atmospheric concentrations of gaseous HONO and the NO3 radical were measured simultaneously for the first time using long path differential optical absorption spectroscopy. Diurnal profiles are reported for two successive nights following days of moderate pollution at Riverside, California, together with concurrent measurements of NO2, O3 and NO concentrations and an upper limit for HCHO levels. These measurements permit an examination of selected aspects of the nighttime atmospheric chemistry of HONO and the NO3 radical and related species. Our data do not support a recently proposed homogeneous gas phase mechanism for HONO formation initiated by the reaction of the NO3 radical with HCHO.