The construction of a differential optical absorption spectrometer (DOAS) is described. The instrument was designed for making automated measurements, with relatively high detection sensitivities, of important tropospheric trace gases, and with the ability to operate in rugged environments without frequent attention. Major innovative features of the instrument include a retroreflector to fold the light path, a diode array detector, and computer‐controlled operation of the entire instrument package. The advantages of these modifications with respect to previously reported designs are discussed in detail. This DOAS has been employed since 1989 to measure NO3 (1 ppt), NO2 (0.6 ppb), HONO, O3, and CH2O (0.8 ppb), where the mixing ratios in parentheses indicate the detection limits over a 5 km path length and 4 min integration time, for the species where these could be determined.