[en] The initial aim of this thesis was to provide the high pressure equipment and the knowledge about the equation of state (EOS) necessary for a research program in a laboratory dealing with spectroscopy of solid hydrogen under high pressure. Once this first goal was reached, a logical step was to extend the work on the EOS to all three hydrogen isotopes and later also to the helium isotpes. During the experiments on the EOS of hydrogen, the effects of the concentration C₁ of the rotationally excited molecules provoked interest, resulting in an extensive experimental and theoretical study. Chapter I describes the results and experience with high pressure equipment for hydrogen up to 7 kbar and chapter II gives a short general introduction to the calculation of the EOS by introducing the Mie-Grueneisen picture and the Silvera-Goldman (SG) potential for hydrogen. Chapter III gives the results of the first EOS of H₂ and D₂ and chapter IV gives a prediction of the EOS of solid T₂ with aid of the SG potential and the experimental results of H₂ and D₂. Chapter V presents calculations on the thermal expansion of the hydrogen isotopes, which are compared with direct experiments and chapter VI deals in detail with the influence of C₁ on the EOS of H₂. Ortho-para conversion in hydrogen is considered in chapter VII, and chapter VIII describes experiments on ⁴He. (Auth.)