The kinetics and mechanism of the reaction of phenylselenenyl halides (PhSeX, X = Cl, Br) and some primary unsaturated alcohols (pent-4-en-1-ol and hex-5-en-1-ol) in tetrachloromethane media have been studied, under the pseudo-first order conditions, in the presence and absence of pyridine as catalyst by variable temperature UV-vis spectrophotometry. Under the kinetic conditions, both the slower uncatalyzed and faster catalyzed (by pyridine) paths give the change of absorbance of the reaction mixture at some wavelength. The obtained values for rate constants have shown that the reactions with phenylselenenyl bromide are slower. Also, the reactions of both phenylselenenyl halides with hex-5-en-1-ol are faster than those with pent-4-en-1-ol. The negative values for entropy of activation for all studied reactions confirm the S(N)2 mechanism of substitution. (C) 2008 Elsevier B.V. All rights reserved.