The solvolysis of ally1 and propargyl chlorides have been carried out in the mixture: formic acid (93,6%)water (6,4%).The relative rates are interpreted in terms of a previously published qualitative general treatment (P. J. C. Fierens, J. Jaminet et P. Kruys, Bull. SOC. Chim. Belges 64, 557 (1955)) based on the competition between two main factors: inductive and mesomeric effects. * * * Kous exprimons notre gratitude a Monsieur le Professeur R. Id. Martin qui a suivi ce travail avec le plus vif intkr&t. (' ) P. J. c. FIERENS, A. IhI.I,EITx et €1. HANNAERT, B d l . S O C . Chim.Belg.. 64, 191 (195.5).