In this study we looked at the mode of action of microwave disinfection. Previous studies have been inconclusive as to whether disinfection occurs due to thermal effects or microwave radiation. To test this we took samples of sterile saline inoculated with one of three common species of bacteria. [italics]Pseudomonas aeruginosa, [italics]Staphylococcus aureus or [italics]Serratia marcescens. The samples underwent one of eight testing protocols. The first two protocols were microwave disinfection at 60ˇ C for 5 min or 10 min cycles. The next two were microwave disinfection at 70ˇ C for 5 min or 10 min cycles. A 2450 MHz, 600 W microwave with a microthermometer was used to maintain constant temperature of the inoculated saline. The other four testing protocols involved heat disinfection of the samples at 60ˇ C for 5 min or 10 min and at 70ˇ C for 5 min or 10 min cycles. A beaker filled with saline was used and temperatures kept constant with a magnetic stirring rod and monitored with a calibrated mercury thermometer. Following disinfection, the samples were plated on Trypticase Soy Blood Agar and colonies counted. Qualitatively there appear to be fewer colonies with microwave disinfection especially after 10 min for all three species of bacteria. Using a Friedman two-way analysis of variance by rank there appear to be no statistically significant difference: [italics]Pseudomonas; 60ˇ C for 5 min (p=.084) or 10 min (p=.046), 70ˇ C for 5 min (p=.318) or 10 min (p=.084), [italics]Staph. aureus; 60ˇ C for 5 min (p=.046), or 10 min (p=.084), 70ˇ C for 5 min (p=.084), or 10 min (p=.084), [italics]Serratia; 60ˇ C for 5 min (p=.158) or 10 min (p=.026), 70ˇ C for 5 min (p=.056) or 10 min (p=.084). It appears that there is more than a thermal effect in microwave disinfection however statistically it is not solely due to microwave irradiation.