Two isomers of vinyl isocyanate and their characteristic features in the room-temperature millimeter wave spectrum. The spectrum reveals compact groups of a-type R-branch transitions of the trans species in the ground state and v₁₈ = 1, v₁₃ = 1, and v₁₈ = 2 excited vibrational states. Weaker a-type R-branch transitions of the less stable cis isomer are, at the present scale, observed in the form of significantly more diffuse groups.