The pseudo first-order rate constants and thermodynamic parameters for the decomposition of cefepime in solid state in absence of humidity and at a relative humidity of 76.4% were calculated. The effect of humidity on the stability of cefepime dihydrochloride monohydrate in the humidity range 25.0%-76.4% at 358 K is described by the equation ln ki = (0.031 +/- 0.0043) x RH% - 10.08 +/- 0.22.