Metal-binding stability constants for a series of N- and N,N'-substituted thioureas with Cu(II), Ni(II), Al(III), and Fe(III) ions were determined by potentiometric titration. The sequence of constants for thiourea, N-methylthiourea, and N,N'-dimethylthiourea indicated steric effects of the methyl groups and that both nitrogen and sulfur were involved in the complexation. The magnitude of the constants was somewhat lower than those of the simple peptides. The mechanism of protection against ionizing radiation by thioureas is probably due to hydrogen-atom transfer rather than binding of metal ions that catalyze cellular oxidations.