1,1-Dichloro-1,3,4,8b-tetrahydroazirino[2,1-a]isoquinolines were prepared by reaction of 3,4-dihydroisoquinolines with dichlorocarbene generated under phase-transfer conditions. The effect of thermolysis conditions on the direction of the aziridine ring opening was studied. Heating the azirinoisoquinolines in an apolar solvent in the presence of a tertiary amine results in the cleavage of the C¹-C⁸ bond to give 2-formyl-1,1-methylene-1,2,3,4-tetrahydroisoquinolines, while thermolysis in the presence of a primary amine involves the formal cleavage of two bonds (C¹-C^8b and C¹-N), yielding the corresponding 3,4-dihydroisoquinolines as major products. When refluxed in alcohols, the azirinoisoquinolines convert to esters of 1,2,3,4-tetraisoquinoline-1-carboxylic acid, and when reacted with amines in the presence of water, to the corresponding amides. Thermolysis of 1,1-dichloro-1,3,3-trimethyl-1,3,4,8b-tetrahydroazirino-[2,1-a]isoquinoline in a polar solvent in the presence of a tertiary amine involves the cleavage of the N-C^8b of the aziridine ring, producing 4,4-dimethyl-1-methylene-2,3,4,5-tetrahydro-1Hbenzazepine-2-one.