The variation of the fracture toughness of MnZn ferrite ceramics with varying loading rate and humidity was determined with the aid of the single edge notched beam (SENB) test. A strong decrease with increasing humidity and decreasing loading rate was observed. A model for subcritical crack growth incorporating kinetic and adsorption effects was formulated to analyze the data. The value of the adsorptioncontrolled fracture toughness was determined independently by double torsion experiments and agreed favorably with the values as determined from the SENB data using the model. The strength of the material was determined, and analysis showed a strength behavior similar to the fracture toughness behavior, as predicted by the model. The analysis presented can be used to assess the subcritical crack growth behavior using a limited number of SENB specimens.