Supersaturated conditions in oceanic and other sedimentary environments, and the crystal morphologies of sedimentary minerals support the occurrence in nature of widespread and powerful crystallization inhibition by extremely small, substoichiometric concentrations (< 1 mg/l) of soluble organic substances. The most powerful inhibitor molecules appear to be polycarboxylic acids, polyphenols, polyphosphates, organic phosphate esters, and hydrolyzed proteins. Organic crystallization inhibition, also referred to as organic adsorption poisoning and threshold inhibition, occurs by a process that is fundamentally different from that of stoichiometric organic complexation. During chemical sedimentation in evaporite and other sedimentary environments, organic inhibition can affect crystallization processes in several distinct but related modes. Highly supersaturated solutions can be stabilized over long intervals, so that eventual destabilization may result in mineral precipitation at unexpected locations. If inhibition is overpowered by excess supersaturation or by partial destruction of inhibitor molecules, the precipitating phase also may be affected. Mineral phase kinetics can be altered so that unexpected mineral species precipitate, and habit modifications, twinning, and intergrowth of precipitates may be induced.