This publication contains recommended methods for estimating specific organic compound vapor emissions from operations handling multicomponent hydrocarbon mixtures (such as crude oils and gasoline). Such an estimate is called speciation. The API Committee on Evaporation Loss Estimation developed this publication. This publication assumes that the user has access to API MPMS 19.1 for Fixed-Roof Tanks@ API MPMS 19.2 for Floating-roof Tanks@ and API Publication 2514A for Marine Vessels@ and uses those to calculate total hydrocarbon emissions. The methods in these publications are used to estimate annual losses from various types of tank construction (including fixed and floating roofs)@ liquid stocks@ stock vapor pressures@ tank sizes@ and wind speeds. They are also applicable for estimating annual losses from loading and unloading operations for various types of marine vessels@ types and volumes of cargo@ and compartment treatment. The methodology in this document applies to: a. liquids with vapor pressure that has reached equilibrium with ambient conditions at a true vapor pressure less than the ambient atmospheric pressure (i.e.@ not boiling). b. liquids for which the vapor pressure is known or for which sufficient data are available to determine the vapor pressure. Speciation of emissions from hydrocarbon mixtures accounts for the preferential evaporation of the more volatile components@ resulting in a different composition of the mixture in the vapor phase than in the liquid phase. The methodology presented in this publication assumes that there is sufficient liquid present such that the chemical composition at the liquid surface may be considered to not change as a result of the evaporative loss. This methodology does not apply to: a. a thin layer of liquid which may readily lose a significant portion of its more volatile components within the period of time for which emissions are to be estimated. b. emissions that result from leaks of a liquid stream (e.g.@ equipment leaks). When a layer of liquid is sufficiently thin that virtually all of the liquid will evaporate within the period of time for which emissions are to be estimated@ then the composition of the vapor phase should be taken as equivalent to the composition of the liquid phase. This is the case for withdrawal losses from floating-roof tanks@ where the withdrawal losses consist of evaporation of the liquid that clings to the tank shell as liquid is withdrawn from the tank. Similarly@ emissions that result from leaks of a liquid stream should be assumed to have the same composition as the liquid phase of the stream.