Thermolysis of azoperester 5 affords gamma-azo radical 14, which cyclizes to hydrazyl radical 15 at a rate of 1.3 x 10(9) M-1 s(-1) at 110 degrees C. Our experimental results are consistent with loss of a methyl radical from 15 to afford 2-pyrazoline 9, which is oxidized in situ by the starting 5 to pyrazole 6. This unusual and endothermic beta-scission can be rationalized if the odd electron in 15 is better aligned with the CH3-C bond than with the weaker t-Bu-N bond. The fact that 5-endo cyclization of 14 is 5 x 10(7) faster than that of the analogous olefinic radical 30 led us to carry out ab initio calculations on simplified structures. Delta H-double dagger for methyl radical addition to diimide is only 0.84 kcal/mol lower than for addition to ethylene and the exothermicity is only 3.5 kcal/mol greater. However, the smaller C-N=N than C-C=C bond angle leads to a Delta H-double dagger 13.3 kcal/mol lower for 5-endo cyclization of 4,5-diazapenten-1-yl than for the analogous 4-pentenyl radical. Photolysis of 5 selectively cleaves the azo group, producing gamma-perester radical 20. This species undergoes intramolecular attack on the peroxide linkage to form lactone 23 at a rate of 1.5 x 10(4) s(-1) at 22 degrees C. The cyclization rate of 20 is slow enough that 5 could be used as a photochemical bifunctional initiator, but cyclization of 14 to the azo group is so rapid that this radical would only rarely attack a monomer.