Systems and methods for the on-site determination of water content in crude oil. The water content of crude oil is determined by mixing a sample of the crude oil with a fixed amount of molecular sieves. The electromagnetic characteristics of the molecular sieves upon mixing with the sample are then measured and used to determine the water content of the sample. This provides a simple approach to measuring the moisture content in crude oil that is extremely fast, accurate, and reproducible without the use of hazardous chemicals.