Lake muds are composed of the organic remains of plants which grew under water in lake or pond systems. Radiocarbon determinations from this type of environment must be analysed with care because of the variable sources of carbon available to the plants which form the gyttja. Sources of carbon include; bicarbonate in the water, dissolved CO2 from the atmosphere, dissolved bicarbonate from limestone and other weathered rocks, old or young organic carbon from humus within the vicinity. These 'inputs' to the lake system may make radiocarbon dates of lake muds problematic. In addition, there may be errors due to post-depositional movement in situ and leaching and adsorption of humic remains in the profile. Humic acids are the decayed remains of old plants. Physical and chemical pretreatment methods usually involving acid-base-acid routines will be required to remove some of these contaminants and validate radiocarbon results.