Coadsorption experiments of alkali metals and hydrogen on Al(100) are reported. The probability for H-H (D-D) dissociation and subsequent atomic adsorption is less than 1:104 at all alkali coverages. As a consequence, each overlayer had to be prepared by exposing the sodium or potassium modified surface to a beam of hydrogen (deuterium) atoms. We observed an attractive alkali-hydrogen interaction at all coverages. This interaction is best described as the formation of an alkali hydride in the presence of excess alkali atoms. The formation of the hydride shifts the recombination and desorption temperature of hydrogen adatoms on Al(100) from around 350 to around 500 K. While no isotope effect was detected on the clean aluminum surface (diffusion limited recombination), a significant 10-15 K shift was observed for both alkali hydrides (bond breaking). Furthermore, both alkali hydrides observed intense vibrational losses, thus revealing the ionic character of the metal-hydrogen bond. Electron energy loss spectra of an annealed monolayer of sodium hydride (deuteride) showed metal-hydrogen stretching bands at 1850 (1350) cm-1 and 1715 (1250) cm-1, a deformation band at 800 (600) cm-1, and a metal-metal band at 200 (200) cm-1. Corresponding peak positions for potassium hydride (deutride) were 1650 (1200) cm-1, 1500 (1100) cm-1, and 775 (585) cm-1. The metal-metal vibration could not be separated from the elastic peak because of the larger mass of potassium compared to sodium.