Averaged deuterations of water (green diamonds), formaldehyde (blue triangles), and methanol (red squares) at 3 × 10⁵ yr (representative of a typical prestellar core age) as function of the total density (abscissa) and temperature (10 K: solid, 20 K: dashed) considering an H₂ opr of 3 × 10^-6 and A_V = 10 mag and considering grain surface parameters as free. Solid boxes refer to observations by Coutens et al. (2012) towards IRAS 16293 for water, and by Parise et al. (2006) (and references therein) towards a sample of low-mass protostars for formaldehyde and methanol.