Eighty Bos taurus crossbred steers sourced from southern Western Australia were monitored to assess the efficacy of electrolyte supplementation on board a livestock vessel travelling to the Middle East during the northern hemisphere summer. Electrolytes (1.8 g/L NaHCO₃ and 3.5 g/L KCl) were added to the drinking water of treatment steers (n = 39) allocated to three pens on the starboard side of the ship. Control steers (n = 40) were allocated to three pens on the port side of the ship. The combined area of the three treatment and three control pens was 61.1 and 63.6 m² respectively, giving a stocking density of 1.57 and 1.55 m² per steer, respectively. Steers were loaded in Fremantle, Western Australia and given 3 days to acclimatise to on-board conditions before being weighed (day 1), after which electrolyte supplementation began while the vessel docked at Port Headland, Western Australia. Feed and water were available ad libitum throughout the experiment. Steers were weighed again on day 18, before discharge in the Middle East. During electrolyte supplementation, wet bulb temperature ranged from 21.3 (day 2) to 31.8°C (day 18). Over the last 3 days of the experiment, wet bulb temperature ranged from 29.0 to 31.8°C with no diurnal variation or night-time cooling. No open-mouth panting was recorded in either group and although animals encountered periods of high heat and humidity (as indicated by increased respiratory rates), the steers were not considered clinically heat stressed during the experiment. After 18 days of electrolyte supplementation, treatment steers had a 2.9 ± 1.7% liveweight advantage compared with control steers (P < 0.001). Urine was collected on days 8 and 16 of the experiment and treatment steers maintained a higher urine pH compared with control steers on both days (day 8; 8.6 v. 8.2 and day 16; 8.2 v. 7.9; P < 0.01). Liveweight advantages and improved regulation of acid–base balance may provide welfare and economic benefits to the live export industry.