NOVELTY - A ratio of methanol to water is determined by establishing a test cell between opposed electrodes contacting the fluid mixture.    USE - For determining the ratio of water to methanol in a reforming feed for producing hydrogen useful in vehicle fuel cell systems.    ADVANTAGE - Because of the greater heat transfer between outer walls of the cells and the fluid in the enclosure when the fluid is a liquid as opposed to a gas, the invention using a reference cell is of greater effectiveness when the mixture being measured is a liquid.    DETAILED DESCRIPTION - Determination of the ratio of a fluid mixture (8) containing methanol and water, involves:    (1) establishing a test cell (3) between opposed electrodes (11, 12, 13) contacting the fluid mixture;    (2) imposing alternating voltage of a given frequency across the electrodes;    (3) measuring dielectric response of the fluid mixture in the test cell at the temperature and frequency;and    (4) determining the ratio of methanol to water in the mixture on the basis of the dielectric response thus measured.    The fluid mixture is tested at a given temperature and the test cell is in open flow communication with the fluid mixture being tested.    An INDEPENDENT CLAIM is included for a sensor for determining a ratio of two compounds in a test fluid mixture, comprising:    (1) a first pair of opposed electrodes establishing between a test cell;    (2) a reference cell (2) established by a second pair of opposed electrodes having equivalent operating characteristics to those of the first pair of opposed electrodes;    (3) an electrical circuit;and    (4) a voltage measuring device connected to the circuit for measuring the voltage drop across the test cell so as to determine the voltage drop across the test cell in relation to the voltage drop across the reference cell.    DESCRIPTION OF DRAWING(S) - The figure shows a vertical cross-sectional view of a sensor.    Reference cell (2)    Test cell (3)    Enclosure (7)    Fluid mixture (8)    Opposed electrodes (11, 12, 13)