This study investigated exposure to heat and freezing of vaccines used in Thailand's National Immunization Program. Cold chain temperatures on 48 randomly selected shipment routes nationwide were monitored. Measles and hepatitis B vaccines were despatched with recording devices and subsequently tested. The study found that extremes of cold appear to be the more significant problem. Heat exposure was relatively brief and not at very high temperatures, so vaccine deterioration was unlikely, as was confirmed by measles vaccine testing. Exposure to temperatures below -0.5 degrees C was widespread, which would be expected to damage hepatitis B vaccine, but shake tests did not detect damage.