It is of interest to know how to obtain as high an absorption efficiency as possible from a co2-absorption canister used in closed- circuit breathing apparatus. In this study the Bureau of Mines found that varying the bed depth and the cross-sectional area of a canister changes it absorption characteristics. In tests using four, rectangular, variable-depth canisters with different cross- sectional areas, the Bureau found that, given a constant weight of the chemical absorbent, lioh, and a constant co2 input, the canisters, with a greater cross-sectional area reached co2 levels of 0.5 Volume-percent in the effluent gas sooner than did the canisters with a greater bed depth. However, resistance to flow through the canister increases with bed depth, and a trade-off is inevitable. It was anticipated that the efficiency of a canister would increase with increasing intergranular air space approaching the tidal volume of the breathing waveform and then level off at some point under 100 percent. This postulation was partially substantiated. Cylindrical and radial canisters were also tested and compared. It appears that shape is not as important a factor in absorption efficiency as bed depth, since neither canister differed significantly from the rectangular canisters with regard to absorption efficiency values.