The nitroxyl-labeled analogues of N,N:N',N':N",N"-tri-1,2-ethanediylphosphoric triamide (TEPA), N,N:N',N'-bis(1,2-ethanediyl)-N"-[[(2,2,6,6-tetramethyl-1-oxypiperidi n-4- yl)amino]carbonyl]phosphoric triamide (5a) and N,N:N',N'-bis(1,2-ethanediyl)-N"-[[(2,2,5,5-tetramethyl-1-oxypyrrolid in-3- yl)amino]carbonyl]phosphoric triamide (11a), possess therapeutic indexes that are 8-12 times higher than those of thio-TEPA (1) and TEPA (2). The introduction of methyl groups into the aziridine ring, or the replacement of the nitroxyl moiety with hydroxylamine or amine derivatives, or with an adamantane moiety, results in compounds of lesser activity. An attempt is made to rationalize these results using a lipophilicity scale. A predictive design pattern is established.