The processes that fix the fractionation of the stable isotopologues of water in the tropical tropopause layer (TTL) are studied using cloud-resolving model simulations of an idealized equatorial Walker circulation with an imposed Brewer-Dobson circulation. This simulation framework allows the explicit representation of the convective and microphysical processes at work in the TTL. In this model, the microphysical transfer of the isotopologues (here, HD16O and H218O) among water vapor and condensed phase hydrometeors is explicitly represented along with those of the standard isotopologue (H216O) during all microphysical interactions. The simulated isotopic ratios of HD16O in water vapor are consistent with observations in both magnitude and the vertical structure in the TTL. When a seasonal cycle is included in the Brewer-Dobson circulation, both the water vapor mixing ratio and the isotopic ratios of water vapor display a seasonal cycle as well. The amplitude and phase of the seasonal cycle in HD16O are comparable to those observed. The results suggest that both the sublimation of relatively enriched ice associated with deep convection and fractionation by cirrus cloud formation affect the isotopic composition of water vapor in the TTL and its seasonal cycle.