The molecular cluster ions HSO4-(H2SO4)n-1(NH3)m corresponding to the neutral species (H2SO4)n(NH3)m for n = 2 to 6 and m = 0 to n-1 have been observed at temperatures up to ~285 K. A transverse chemical ionization apparatus was located inside a cooled flow tube where water, sulfuric acid, and ammonia vapors mixed and formed clusters. The complexities of the experimental technique and the interpretation of the results are extensively discussed. Typical NH3 and H2SO4 concentrations were 2 ¿ 109 cm-3, i.e, ~100 pptv at atmospheric pressure. For these conditions, cluster concentrations were estimated to be a few times 106 cm-3 and the critical, particle-forming cluster likely contained 2 H2SO4 molecules at 275 K. The results are consistent with the species (H2SO4)2NH3 playing an important role in the formation of new particles in the atmosphere.