TPD spectra (m/z = 30) for the H₂CO:H₂O mixtures, a) 1:14 deposited at 15 K, heated at 2 K min^-1 90 − 210 K, b) 1:8 deposited at 15 K, heated at 2 K min^-1 90 − 135 K, held at 135 K for 14.5 h, heated at 2 K min^-1 135 − 210 K, c) 1:6 deposited at 90 K, heated at 2 K min^-1 90 − 210 K, d) 1:9 deposited at 90 K, heated at 2 K min^-1 90 − 135 K, held at 135 K for 18 h, heated at 2 K min^-1 135 − 210 K. The arrows indicate that an isothermal break of several hours was made in the TPD experiment. M, V, and C correspond to the multilayer, the volcano and co-desorption desorptions, respectively. The asterisk indicates a double peaked structure, observed as a result of desorption of H₂CO from the radiation shield.