The Vacuum Ultraviolet spectrum of HD and D2 has been studied under high resolution and is compared to that of H2. Three band systems, B1Σ+u - X1Σ+g, C1Πu -X1Σ+g and B'1Σ+u - X1Σ+g were observed up to the dissociation limit. In HD the forbidden transitions EF1Σ+g - X1Σ+g, G1Σ+g - X1Σ+g, and I1Πg - X1Σ+g were also observed for the first time.
The analysis of these band systems gave accurate rotational and vibrational constants for the excited states. A comparison of these constants is made with those derived from ab initio calculations and the deviations are discussed. An electronic isotope shift was observed in the B1Σ+u and C1Πu states, and the adiabatic corrections at the equilibrium position were calculated. The nature of the forbidden transitions and the correlation of the six excited states (B, C, B', EF, G, and I) to the two dissociation limits in HD is discussed.