The performance of CsI photocathodes has been characterized for use with grazing incidence soft x-rays. The total electron yield and pulsed quantum efficiency from a CsI photocathode has been measured in a reflection geometry as a function of photon energy (100 eV to 1 keV), angle of incidence and the electric field between the anode and photocathode. The total electron yield and pulsed quantum efficiency increase as the x-ray penetration depth approaches the secondary electron escape depth. Unit quantum efficiency in a grazing incidence geometry is demonstrated. A weak electric field dependence is observed for the total yield measurements; whilst no significant dependence is found for the pulsed quantum efficiency. Theoretical predictions agree accurately with experiment.