A petroleum stream microwave water cut monitor includes test cell means which contains a reference petroleum multiphase fluid sample and which has a sample stream of a petroleum stream passing through it. A source transmits microwave energy to one of a first pair of antennae which irradiates the petroleum stream flowing in the test cell or the reference sample in the test cell with microwave energy. One of a second pair of antennae receives the microwave energy that has passed through either the petroleum stream or the reference sample. A detector detects the received microwave energy and provides a signal corresponding thereto. An indicator provides an indication of the water cut of the petroleum stream in accordance with the received signal power and a phase difference between the transmitted microwave energy and the received microwave energy.