We report Grand Canonical Monte Carlo (GCMC) molecular simulation studies of argon, nitrogen and water in model porous carbons. Adsorption in "buckytubes" is studied for argon and nitrogen, We calculate adsorption isotherms, density profiles, phase transitions and heats of adsorption for model tubes of 1.05 and 4.8 nm diameter at 77 and 60 K. The smaller buckytube is too small to exhibit phase transitions. For the larger tube we observe layering transitions, capillary condensation and hysteresis for both argon and nitrogen. Fluid molecules are modelled as Lennard-Jones spheres, and an averaged fluid-wall potential, dependent only on the distance of the adsorbed molecule from the centre of the tube, is used. Results are reported for water in model graphitic and activated carbons with slit pores. Water is modelled using the TIP4P potential, and COOH groups are used as the active sites, the water-COOH interaction being the OPLS potential. The structure of the carbon walls is taken into account. We study the effect of varying the density and arrangement of active groups on the surface. For these carbons the adsorption is generally greater than for graphitic carbons, and is strongly dependent on the arrangement of the active groups on the surface.