Oxygen isotopic selectivity on infrared multiphoton dissociation of 2,3-dihydropyran has been studied by the examination of the effects of excitation frequency, laser fluence, and gas pressure on the dissociation probability of 2,3-dihydropyran and isotopic composition of products. Oxygen-18 was enriched in a dissociation product: 2-propenal. The enrichment factor of O-18 and the dissociation probability were measured at a laser frequency between 1033.5 and 1057.3 cm(-1), the laser fluence of 2.2-2.3 J/cm(2), and the 2,3-dihydropyran pressure of 0.27 kPa. The dissociation probability decreases as the laser frequency being detuned from the absorption peak of 2,3-dihydropyran around 1081 cm-1. On the other hand, the enrichment factor increases with detuning the frequency. The enrichment factor of 110 increases with increasing the 2,3-dihydropyran pressure at the laser fluence of 2.7 J/cm(2) or less and the laser frequency of 1033.5 cm(-1), whereas the yield of 2-propenal decreases with increasing the pressure. A very high enrichment factor of 751 was obtained by the irradiation of 0.53 kPa of 2,3-dihydropyran at 2.1 J/cm(2). Collisional effect of vibrationally excited molecules with ambient molecules on isotopic selectivity is discussed on the basis of a rate equation model including a collisional vibrational de-excitation process.