The adsorption states of hydrogen on Pt(100)-(hex) and (1 x 1) surfaces have been studied using TDS and HREELS in the temperature range of 170-330 K. Hydrogen atoms are shown to adsorb on the (1 x 1) surface in bridge and 4-fold hollow sites. On the (hex) surface hydrogen adsorption induces a partial lifting of the (hex) reconstruction with the consequent appearance of the bridge and 4-fold hollow states on (1 x 1) patches. Additional adsorption states appear due to the population of structural defects and patches of the (hex) phase.