Microstructures, phases, and phase transformations in Ti-6Al-4V alloy specimens containing 0, 10, 20, and 30 at. pet hydrogen were investigated using optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and microhardness testing. Alloying with hydrogen was achieved by holding the specimens in a pure hydrogen atmosphere of different pressures at 780 degreesC for 24 hours. The phases present in the temperature range of 20 degreesC to 1000 degreesC were determined by microstructural characterization of the specimens quenched from different temperatures. Increasing the hydrogen addition from 0 to 30 at. pet lowered the beta-transus temperature of the alloy from 1005 degreesC to 815 degreesC, significantly slowed down the kinetics of the beta-to-alpha transformation, and led to formation of an orthorhombic martensite instead of the hexagonal martensite found in quenched specimens containing 0 pet H. A hydride phase was detected in specimens containing 20 and 30 at. pet hydrogen. The time-temperature-transformation (TTT) diagrams for beta-phase decomposition were determined at different hydrogen concentrations. The nose temperature for the beginning of the transformation decreased from 725 degreesC to 580 degreesC, and the nose time increased from 12 seconds to 42 minutes when the hydrogen concentration was increased from 0 to 30 at. pet.