In this paper we present a novel technique for experimental determination of dew and bubble points of oils and gases under reservoir conditions. Extrinsic fiber optic sensors mounted on an 80-mL titanium equilibrium cell are part of the measurement setup. Formation of a new phase in the sample confined within the cell is sharply indicated by the creation of characteristic changes in the fiber optic signal. This allows for precise detection of dew and bubble point conditions. The entire apparatus is transportable and can easily be carried to field locations for quick and accurate studies of bottom hole samples. Phase behavior measurements (dew and bubble points) of three model systems and a real reservoir fluid are presented.