Trans-cis isomerisation of the azo amphiphilic derivatives 4-(N-methyl-N-n-dodecyl)amino-4′-(N-thiazol-2-yl) sulfonamidoazobenzene (1) and 4-(N-methyl-N-n-dodecyl)amino-4′-(N-pyrimidin-2-yl)sulfonamidoazobenzene (2), each containing a sulfonyl group and a heterocyclic ring, was observed during irradiation of the amphiphile solutions at the absorption band maximum (ca. 450 nm). The reverse cis-trans transition occurred at room temperature as a thermal relaxation. Quantum chemical calculations yielded the optimum geometry of the trans, cis and transition forms. The trans-cis potential energy difference obtained by the DFT B3LYP method is in the range of 67.5–70.7 kJ mol^-1 and the activation energy is ca. 129.7–132.5 kJ mol^-1, depending on the basis set. The UV-Vis spectra of the cis and trans form of 1 and 2 were calculated using the INDO1/S semiempirical program. The calculated spectra of the cis-trans mixtures and those obtained experimentally were very close in shape. For the alcanoyl derivative 4-(n-undecyl)carbonamido-4′- (N-thiazol-2-yl)sulfonamidoazobenzene (3) the illumination with low intensity monochromatic light (365 nm) caused isomerisation but the system needed up to 72 h to recover the trans form by thermal relaxation.