The role of aryne intermediates in the reactions of acetylarylnitrosamines in solution has been investigated. Participation by 3-t-butylbenzyne in the decomposition of o-t-butyl-N-nitrosoacetanilide in benzene, leading to m-t-butylphenyl acetate, has been confirmed, although the major product, o-t-butylphenyl acetate, is formed predominantly from o-t-butylphenyl carbonium ions, 5-t-Butyl 1,4-dihydronaphthalene-1,4-endoxide has been isolated from the decomposition in the presence of furan. In common with the o-isomer, m- and p-t-butyl-N-nitroso-acetanilides yield 'aryne adducts' with 2,3,4,5-tetra- phenylcyclopentadienone in benzene. They do not, however, form t-butyl-l,4-dihydronaphthalene-l,4-endoxides with furan, and participation by a true aryne in their reactions and in those of unsubstituted N-nitrosoacetanilide, is discounted. The nature of the arynoid intermediato has not, in spite of the elimination of many possibilities, been conclusively established, but it is probably the dipolar conjugate base formed by removal of an o-proton from the arenediazonium cation. Anomalies similar to those observed in the decomposition of o-t-butyl-N-nitrosoacetanilide are exhibited by 2,5-di-t-butyl-N- nitrosoacetanilide, which affords aryne adducts with both furan and anthracene in greater yield than does the mono-substituted nitrosamide. Aryne participation was not evident in the highly complex decompositions in benzene of 1,4-di-t-butyl-2,5-di-(N-nitrosoacetamido) benzene, 2,5-di-t-butyl-2,5-di-(N-nitrosoacetamido)benzene, and 4-acetoxy-2,5-di-t-butyl-N-nitrosoacetanilide, Formation of 2,5-di-t-butylphenol, 2,5-di-t-butyl-2-benzoquinone, and acetic anhydride in the last-named reaction indicates deacetylation of the intermediate acetoxybenzenediazonium acetate, and suggests that complications in the reactions of the dinitrosamides arise from interaction between the N-nitrosoacetamido substituents. The decompositions of diphenyl[o-(N-nitrosoacetamido)phenyl] phosphine oxide and diethyl o-(N-nitrosoacetamido)phonyl]phosphonate have been studied, and evidence of aryne intermediacy obtained. The results are discussed in terms of currently acceptable mechanisms for the decomposition of N-nitroso- acetanilide and, where necessary, modifications to these are suggested.