The formation of organophosphate molecules by prebiotic processes relies on nonenzymatic synthesis. Given the centrality of phosphorylated biomolecules in metabolic, structural, and replicative processes, it is highly likely that such nonenzymatic synthesis had to occur early in Earth's history. This Review collects and uses thermodynamic data to constrain processes that may have produced organophosphates and evaluates both the plausibility of reactants and the likelihood that environments conducive to phosphorylation were present. The energy required to phosphorylate organics is ∼15 kJ/mol, requiring either very low water activities or reactive inorganic phosphorus compounds. Thermodynamics permits evaluating phosphorylation environments for both plausibility and novelty and shows that several routes would have been available to form these potentially key reagents. Building from phosphate monoesters to diesters may have enabled the synthesis of nucleic acids, perhaps opening a way into the RNA world.