The critical temperatures and pressures of caprolactam (CASRN 105-60-2), dimethyl sulfoxide (CASRN 67-68-5), 1,4-dimethylpiperazine (CASRN 106-58-1), and 2,6-dimethylpiperazine (CASRN 108-49-6) have been measured by the pulse-heating technique applicable to thermally unstable compounds. Experimental critical constants have been compared with those calculated using the group contribution methods of Wilson and Jasperson, Marrero and Gani, Hukkerikar and co-workers. The critical properties of dimethyl sulfoxide measured in this work have also been compared with the results of computer simulation by Vahid and Maginn. (C) 2018 Elsevier B.V. All rights reserved.