A compilation of information relating to substrate metabolism via human cytochromes P450 (CYP) from the CYP1 and CYP2 families is reported. The data presented include details of preferred sites of metabolism and Km values (usually for the expressed enzymes) for each reaction for selected substrates of CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6 and CYP2E1. Although other P450 databases are available, they do not provide such information as is collated here, and which can prove useful for comparing P450 substrate characteristics. This information can be employed in analysing the structural requirements for human P450 enzyme selectivity and for establishing various rules regarding preferred site of metabolism for selective P450 substrates. For example, in most cases it would appear that there is a set number of intervening 'heavy' atoms (atoms other than hydrogen) between sites of metabolism and key hydrogen bond acceptors (or donors) for human P450 substrates, with the number of intervening atoms being dependent upon the type of P450 involved.