The 70 eV electron ionization (EI) and chemical ionization (CI) mass spectra were recorded for six differently substituted 2-benzothiazolphenylhydrazones and five related 1-(2-benzothiazolyl)-3,5-diphenylformazans. The fragmentation patterns were studied by exact mass measurements, metastable ion analysis and collision-induced dissociation techniques. For both hydrazones and formazans the EI spectra were relatively simple, and especially for the formazans the dominance of the C3-N4 bond cleavage was striking. The CI spectra of the formazans were recorded with the use of ammonia, d 3 -ammonia, isobutane or methane as reagent gas. Only methane was used for the hydrazones because even it was able to cause only little fragmentation. In comparison with the hydrazones, the protonated formazans were relatively unstable, decomposing through several fragmentation pathways. Regardless of the reagent gas, the decomposition was always of the same degree and the spectra were closely similar. The most probable protonation site for the formazans was estimated by semiempirical calculations to be the benzothiazole nitrogen. This result is in agreement with the experimental data under both EI and CI, and showed that those tautomeric forms of formazans having a single bond between C3 and N4 are dominant in the gas phase.