The microwave spectra of ten isotopic species of nitrosomethane have been measured enabling structures for CH₃NO and CD₃NO to be determined independently. The mean of these structures is found to be : NO = 1.211, C—N = 1.480, C—H_i(in plane)= 1.094, C—H₀(out-of-plane)= 1.094 Å, ∠CNO = 113.2, ∠H_iCN = 111.0, ∠H₀CN = 107.2 and ∠H₀CH₀= 109.2°. The methyl group is tilted by 2.5° away from the oxygen atom. The orientation of the dipole moment in nitrosomethane has been determined from isotopic measurements. For CH₃¹⁵NO, higher order Stark perturbation terms, arising through an accidental near degeneracy, had to be included to obtain an accurate dipole moment. The components of the dipole moment are; µ_a= 2.262, µ_b= 0.516 and µ= 2.320 ± 0.004 D. The orientation of the dipole moment in the related molecule acetaldehyde has been determined from isotopic work and previous Stark effect measurements have been refined to give µ_a= 2.537, µ_b= 1.062 and µ= 2.750 ± 0.006 D.

A, E splittings in the microwave spectra of CH₃NO and CD₃NO have been analysed by both the principal axis method (PAM) and the internal axis method (IAM) for a rigid top–rigid frame model. An attempt to fit the spectrum of CD₃NO was made using Woods' IAM treatment in conjunction with Watson's centrifugal distortion theory. More precise distortion constants were then obtained from the spectra of sym-CH₂DNO and sym-CHD₂NO and used to determine a preliminary harmonic force field for nitrosomethane. A few perturbations in the spectra of the symmetric species have been observed due to sym–asym interactions. The structure, bonding and vibrational properties of nitrosomethane are discussed.