Thiourea/alkylamine cocatalysts have previously been shown to be effective systems for the ring-opening polymerization (ROP) of lactide, but an experimental explanation for the varied activity and selectivity of these structurally similar alkylamine cocatalysts combined with thiourea is elusive. In this work, several alkylamine bases are shown to be weakly associated with a thiourea cocatalyst in solution, and the nature of cocatalyst interactions vary with the identity of the alkylamine. Kinetic analyses of the organocatalytic ROP reactions reveal noninhibitory behavior in [alkylamine] and a new mode of activity for thiourea. Reactivity patterns are proposed based on computed cocatalyst geometries, and a new cocatalyst pair for the ROP of lactide is disclosed.