Respiration rate (R O2 ) of prickly pear cactus stems (Opuntia spp.) was measured as a function of 4 temperature (T) and 6 relative humidity (RH) combinations for O 2 partial pressures between 15 and 0.8 kPa, which were considered to support aerobic respiration. The rate of respiration (R O2 ) was determined based on O 2 depletion of the atmosphere in sealed containers containing 1 kg of stems. The O 2 partial pressure declined linearly over time and the slopes of the fitted lines were used to calculate the rate of O 2 uptake. The rate of O 2 uptake increased with increasing temperature and decreased with increasing RH. The respiratory rate at 25°C was approximately 30 to 40 times higher than at 5°C. The respiratory rate at 65% RH was between 30 and 90% greater than at 90% RH, depending on the temperature. Data for ln(R O2 ) for each RH level were regressed against the inverse of the T (K -1 ) to determine Arrhenius constants and calculate the apparent Ea of respiration for the six RH conditions. The Ea was similar for each RH level, varying between a low of 113 to a high of 131 kJ·mol -1 . An equation having an R 2 of 0.95 was developed describing respiration as a function of RH and T (°C) using only four constants.