The polycyclic nitramine CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane) is being considered for use as a munition, but its environmental fate and impact are unknown. The present study consisted of two main elements. First, sorption–desorption data were measured with soils and minerals to evaluate the respective contributions of organic matter and minerals to CL-20 immobilization. Second, since CL-20 hydrolyzes at a pH of >7, the effect of sorption on CL-20 degradation was examined in alkaline soils. Sorption–desorption isotherms measured using five slightly acidic soils (5.1 < pH < 6.9) containing various amounts of total organic carbon (TOC) revealed a nonlinear sorption that increased with TOC [K
d (0.33% TOC) = 2.4 L kg−1; K
d (20% TOC) = 311 L kg−1]. Sorption to minerals (Fe2O3, silica, kaolinite, montmorillonite, illite) was very low (0 < K
d < 0.6 L kg−1), suggesting that mineral phases do not contribute significantly to CL-20 sorption. Degradation of CL-20 in sterile soils having different pH values increased as follows: sandy agricultural topsoil from Varennes, QC, Canada (VT) (pH = 5.6; K
d = 15 L kg−1; 8% loss) < clay soil from St. Sulpice, QC, Canada (CSS) (pH = 8.1; K
d = 1 L kg−1; 82% loss) < sandy soil provided by Agriculture Canada (SAC) (pH = 8.1, K
d = approximately 0 L kg−1; 100% loss). The faster degradation in SAC soil compared with CSS soil was attributed to the absence of sorption in the former. In summary, CL-20 is highly immobilized by soils rich in organic matter. Although sorption retards abiotic degradation, CL-20 still decomposes in soils where pH is >7.5, suggesting that it will not persist in even slightly alkaline soils.