2-Nitrosopyridine, 1, 3-methyl-2-nitrosopyridine, 2, and 4-methyl-2-nitrosopyridine, 3, exist in organic solvents as monomer-azodioxy dimer equilibria with the dimers predominating at ambient temperatures. In the case of compounds 1 and 3 only the Z-dimers co-exist with the monomers, whereas for compound 2 both Z- (major) and E- (minor) dimers are present with the monomer. Variable temperature H-1 NMR bandshape analysis and 2D-EXSY spectra of 1-3 provided thermodynamic and kinetic data for the dissociation equilibria Z- (or E-) dimer reversible arrow 2 monomer, DeltaH(empty set) values being in the range 53-58 kJ mol(-1) and DeltaG double dagger (298.15 K) values in the range 70-82 kJ mol(-1). An X-ray crystal structure of 1 identified the compound as a Z-dimeric species with the two pyridine rings twisted by 62.7 degrees and their heterocyclic N atoms pointing towards each other.