The mass spectra of a series of benzo[c]cinnoline derivatives are reported and discussed. The spectra are useful for analytical purposes, as all compounds give pronounced molecular ions, and the fragmentation processes generally occur without skeletal-rearrangement. Many benzo[c]cinnolines which are substituted in the 4-position may be differentiated from other isomers by the presence of “proximity-effects” in their mass spectra. © 1968 CSIRO. All rights reserved.