A known amount of marker may be added to a pressurized hydrocarbon fluid, or combined with an additive to form a mixture, and the mixture may be added to a pressurized hydrocarbon fluid. An amount of the marker in the hydrocarbon fluid may be determined. An amount of the additive in the hydrocarbon fluid may be determined based on the amount of the marker in the pressurized hydrocarbon fluid. An inline system may be used for detection of a marker in a pressurized hydrocarbon fluid. The system is fixed in a pressurized hydrocarbon fluid supply line such that pressurized hydrocarbon fluid flowing from a first location to a second location in the supply line passes through the detection system. The hydrocarbon fluid may be a liquid or a gas. The markers may include a variety of optical markers, such as fluorescent markers. Detection of the markers may include, for example, fluorescence detection.