The [3 + 2] and [3 + 3] cyclodimerisation processes of small nitrile oxides, XCNO (X = F, Cl, Br, CN, CH3) are investigated by ab initio coupled cluster theory at the CCSD, CCSD(T) and MR-AQCC levels for the first time. The favoured dimerisation process is a multi-step reaction to furoxans (1,2,5-oxadiazole-2-oxides) involving dinitrosoalkene-like intermediates with diradical character. The rate determining step for all but the F-species is the first, corresponding to the C–C bond formation. The kinetic energy barrier depends on the nature of the substituent X, generally increasing with decreasing electronegativity and increasing π-donor ability of the substituent: F (ΔG298 = 0 kJ mol−1) < Cl (72) < Br (90) < CH3 (104) < CN (114) (MR-AQCC(2,2)//UB3LYP/cc-pVTZ). Following initial C–C bond formation, three possible dinitrosoethylene diradical pathways are explored. Two of them are new, and one of them is a low-energy three-step path with implications for cycloreversion, tautomerism and detection of dinitrosoethylene intermediates. Alternative one-step, concerted [3 + 2] and [3 + 3] cyclodimerisation processes leading to 1,2,4-oxadiazole-4-oxides and 1,4,2,5-dioxadiazines have kinetic energy barriers around 100–240 kJ mol−1 (CCSD//B3LYP), some 1.6 to 2.5 times higher than those leading to furoxans, supporting the experimental observations of furoxan formation as nitrile oxide loss channels during storage, trapping/re-vaporisation and reactions of nitrile oxides. Potential polymerisation initiation processes for NCCNO, involving the 1,2-dipolar NC substituent are also explored.