The traditional view that landfill waste is a long-term pollutant problem has been investigated as part of an evaluation of the properties of mature municipal solid waste (MSW) (>30 years old). This evaluation has shown that following the active biological period, the nature of the landfill waste mass has evolved from a pollutant source into a pollutant sink. The distribution coefficients (Log Rd) between the solution and the solid for Cd and Zn are in the order of Log 2.4 and maximum values of up to Log Rd 3.3 are reported here. This demonstrates strong net partitioning to the solid phase from the aqueous phase, over and above that retained as part of the primary landfill source material. The sorption characteristics appear to be due to an almost unique formation of a soil-like substance with significant calcium carbonate and stable organic matter content, interspaced with non-degradable material. Although sulphide minerals were not identified directly from the waste samples characterized, their presence is suspected from the methanogenic conditions the waste was subjected to during the biologically active lifetime of the site. The resultant sorption capacity appears to be related to the residual biological mineral formation that provides both pH buffering and available adsorption surfaces. This pH buffering capacity also appears to compensate for any secondary release of metals from sulphide oxidation under aerobic conditions driving the re-adsorption to other mineral surfaces. This detailed evaluation has shown that the long-term evolution of the site and subsequent reactions are a function of the biological processes that the waste was exposed to. The controlling factors for long-term pollutant release are therefore related to the substantial primary organic content degraded.