Photodediazoniations, using a high pressure mercury lamp, of p-N,N-dimethylaminobenzenediazonium salts proceed by a heterolytic mechanism via the formation of a triplet p-N,N-dimethylaminophenyl cation. The triplet state cation has been shown to be more stable than the singlet by Gaussian 70 molecular orbital calculations. The product of the photodediazoniation of p-N,N-dimethylamino-benzenediazonium hexafluorophosphate in the solid state was proposed to be a 1:1 p-N,N-dimethylfluoroaniline phosphorus pentafluoride adduct, and formed from an ion-pair precursor in a concerted reaction. The photodediazoniation of p-N,N-dimethylaminobenzene-diazonium salts in acetonitrile and 2,2,2-trifluoroethanol solutions, yielded products consistent with heterolytic mechanisms already accepted for the thermal dediazoniation of other aryl diazonium salts in these solvents. A heterolytic mechanism via the formation of a carbonylic adduct is proposed for the photolysis in acetone solution, while in 1.1.3.3-tetramethylurea and 1-methyl-2-pyridone solutions the isolation of the novel carbonylic adducts 2-(p-N,N-dimethylaminophenoxy)-1.1.3.3-tetraraethylamidinium hexafluorophosphate and 2-(p-N,N-dimethyl-aminophenoxy)-1-methylpyridinium hexafluorophosphate respectively was achieved. The photolysis of other diazonium hexafluorophosphates having absorption bands in the range 373 - 411 nm also yielded analogous carbonylic adducts in these solvents, while the scope of these reactions was expanded to include hexafluoroantimonate and hexachlorostannate diazonium salts. Thermal dediazoniation of ortho-methyl substituted benzenediazonium salts in the carbonylic solvents 1.1.3.3-tetraraethylurea, 1-methyl-2-pyridone, 1,3-dimethyl-2-imidazolidinone, and cyclohepta-2,4,6-triene-1-one (tropone) were shown to yield isolatable carbonylic adducts in parallel with the photo-dediazoniations. The ease of hydrolysis of the novel carbonylic adduct (2',6'-dimethylphenoxy)tropylium hexafluorophosphate, indicated that adducts formed on dediazoniation in acetone solution should be highly susceptible to hydrolysis. From Gaussian 76 molecular orbital calculations, the schematic energy profile for the Na, N[beta] rearrangement in the 2,6-dimethylbenzenediazonium ion was shown to be very similar to that for the benzenediazonium ion.