Aromatic amines have been used as electron transfer probes for the cytochrome P450s and other enzyme systems. If electron transfer occurs, the radical cations generated from these compounds are expected to undergo ring opening, in direct analogy to the cyclopropylcarbinyl → homoallyl neutral free radical rearrangement, generating a carbon-centered radical which will disrupt the enzyme active site and lead to inactivation. Indirect evidence and MO calculations imply that this assumption may be valid for aliphatic amines, although no absolute rate constants are known. Considerably less is known about the behavior of aromatic amines such as N-cyclopropyl-N-methylaniline and derivatives which have been used as substrates to probe the chemistry of the cP450 family of enzymes. Recent examinations show that the rate constant for ring opening of N-methyl-N-cyclopropylaniline is 4.1 x 10⁴ s^-1. These results are best explained two phenomena (Scheme 1): (i) a resonance effect in which the spin and charge delocalization stabilizes the radical cation in the ring closed form, and (ii) the lowest energy conformation of the molecule does not meet the stereoelectronic requirements for ring opening. This presentation will focus on the mechanism and kinetics of ring opening of cyclopropyl amminium radical cations of direct relevance to enzymatic oxidations. Radical cations generated from N-cyclopropylanilines, and subsequent examinations by electrochemical methods and nanosecond laser flash photolysis will be discussed.