Adsorption isotherms in the pressure range 10(-11)-10(-6) Torr have been measured for H-2 and mixtures of H-2 and CH4, CO, and CO2 on copper plated stainless steel at 4.2 K. The measurements have been focused on the behavior of the isotherms at low surface coverage, up to a few monolayers of adsorbed gas on the cold surface. The isotherms were measured in a static situation with a small amount of gas injected for each point on the isotherm. Coadsorption measurements of H-2 with CH4, CO, and CO2 as well as adsorption of H-2 On condensates of CH4, CO, and CO2 have been made. A cryotrapping effect of H-2 is seen when coadsorbing the gas mixtures, which is especially strong for the mixture of H-2 and CO2. The measurements show that CO2 condensed at 4.2 K may have a porous structure that H-2 can penetrate, while CO and CH4 have rather dense structures when adsorbed at 4.2 K. The measurements have been done within the framework of the Large Hadron Collider project [The Large Hadron Collider Accelerator Project, CERN/AC/95-05 (LHC), 20 October 1995] at CERN, Geneva, Switzerland, where superconducting magnets will be used to produce a magnetic field of about 9 T around the arcs of the 27 km long quasicircular accelerator. Synchrotron radiation produced by the circulating proton beam in the superconducting magnets will induce desorption of neutral gas molecules, mainly H-2, CH4, CO, and CO2, from the inner surface of the vacuum chamber. The desorbed gas molecules will be physisorbed on the cold surfaces in the vacuum chamber and they may induce vacuum instabilities in the accelerator. Hence, it is of importance to have knowledge of the mixed adsorption isotherms of these gases at low temperatures.