[CH₂DOH]/[CH₃OD] ratio as a function of the protostar luminosity. Low-mass data (IRAS 16293, IRAS 4a, IRAS 4b and IRAS 2) were taken from Parise et al. (2006), while the data for Orion IRc2 is from Jacq et al. (1993). The upper limit reported in Table 1 for G24.78+0.08 is not plotted because of the uncertain identification of the CH₃OD line at 133.9254 GHz. The horizontal dashed line refers to the value predicted by grain chemistry models (Charnley et al. 1997).