The five crystal phases of cephalexin were prepared and characterized by infrared spectrum, differential thermal analysis (DTA) and X-ray diffraction. Phase I (anhydride) was obtained by drying at 130°C under vacuum in P₂O₅ desiccator. Phase II (dihydrate) was obtained by absorbing water under 95% of relative humidity (R.H.). Phase III (demethanolate) was obtained by storage under vacuum in P₂O₅ desiccator. Phase IV, the most stable monohydrate, was obtained by patent preparation. Phase V (deacetonitorilate) was obtained by heating at 40°C under vacuum. Phase I and phase II were stored under 0-95% of R.H. at 35°C for 2 weeks. When phase I was stored under more than 20% of R.H., it absorbed about I mol equivalent of water and was transformed into the monohydrate (phase IV). When phase I was stored under 95% of R.H., it was transformed into the dihydrate (phase II). When phase II was stored under less than 62% of R.H., it lost about 1 mol equivalent of water and was transformed into phase IV. When phase II was stored under 0% of R.H., it was transformed into phase I (anhydride). When the most stable phase IV was stored under 0% of R.H. without heating, it lost about 1 mol equivalent of water, and was transformed into phase IV' (anhydride). Phase III absorbed about 1/2 mol equivalent of water under 11-62% of R.H. When phase III was stored under 95% of R.H., it absorbed more than 2 mol equivalent of water, and was transformed into phase II. Phase V absorbed water proportional to R.H. under more than 20% of R.H.