The concentrations of water dimer are compared in Ne and p-H-2 matrices at low temperatures, using infrared spectroscopy. Additional data are given for o-D-2 and Ar matrices. For a given monomer concentration, the dimer concentration is significantly higher in solid Ne (or Ar) than in solid p-H-2. In p-H-2, the dimer concentration is only slightly higher than expected for a random distribution of water in the matrix. The dimer concentration in o-D-2 matrices is intermediate between p-H-2 and noble gas matrices. This strongly suggests that most dimers form on the surface of the growing matrix, and not as the result of diffusion in the bulk of the matrix.