Cooperative upconversion processes and Yb-Er energy transfer efficiencies in high Er concentration phosphate glasses were studied. The cooperative upconversion coefficients were deduced from the pump intensity dependence of luminescence decay curves. Cooperative upconversion coefficients of 4I13/2 level, for Er3+ concentrations higher than 1 X 1020 cm-3, are one order of magnitude smaller than the ones reported for silica glass. The increase in the cooperative upconversion coefficient with the increase in Er3+ concentration was found to be small and Er3+ concentrations as high as 3.7 X 1020 cm3+ in this glass are feasible. Yb-Er energy transfer efficiency in Yb/Er co-doped phosphate glasses, with Er concentrations as high as 1.9 X 1020 cm-3 and 2.4 X 1020 cm-3, were measured with a pump and probe technique and also estimated from lifetime measurements. The energy transfer efficiencies exceed 95 percent, although the ratio of the concentrations, Yb/Er, is only about 1.2 and 2 in the samples studied.This confirms that efficient pumping of high Er3+ concentration phosphate glass, required in waveguide amplifiers, can be achieved utilizing Yb/Er co-doping.