We have compared the performance of CF 3I, CD 3I, and CH 3I in an atomic iodine photodissociation laser over the pressure range 1–200 torr. At pressures below 5 torr, CD 3I produces larger energy outputs, while above 5 torr CF 3I gives superior performance. The crossing of the laser energy output versus pressure curves is explained on the basis of collisional quenching of I( 2P 1 2 )(≡I *) by undissociated alkyl iodide.