A refined biofilm formation method was used to evaluate the stability of a simulated liquid waste form containing a simulated liquid waste (salts) and cement in three different proportions, and a simulated solid waste form containing a simulated solid waste (resin) and cement in three different proportions. The experimental samples of all the simulated liquid waste forms showed evidence of microbial growth on them after 3 days of evaluation as indicated by substantial increase in sulfate production, and exhibited considerable instability to microbial degradation as indicated by substantial leaching of calcium. The experimental samples of all the simulated solid waste forms showed evidence of inhibition of growth of Thiobacillus thiooxidans for about 18 days, after which the growth of the microbe became evident in two out of three. Within the growth inhibition period, the differences between experimental and control samples were minor. After the growth of T thiooxidans became evident, comparatively higher degradations were observed for the experimental samples of the resin containing solid waste forms. (C) 2002 Elsevier Science B.V. All rights reserved.