We compared the effects on microvascular reactivity of hydroxyethylstarch (Volulyte^®) and gelatin (Geloplasma^®) during acute haemodilution. The hypothesis was that Volulyte would provide better microvascular reactivity than Geloplasma. Forty patients undergoing elective cardiac surgery were randomly assigned to receive either Volulyte or Geloplasma as the exclusive priming solution of the cardiopulmonary bypass. To evaluate microvascular reactivity, postocclusive reactive hyperaemia was examined before and after cardiopulmonary bypass. Microvascular reactivity assessments included the rate of the occlusion and reperfusion slopes and reperfusion times. After cardiopulmonary bypass, increases in reperfusion time were significantly smaller in the Volulyte group (3 (−27 to 9 [−35 to 33]%) vs 29 (−17 to 76 [−34 to 137]%) in the Geloplasma group, p = 0.02 between groups). Rate of reperfusion increased in the Volulyte group (26 (−17 to 43 [−59 to 357])%), whereas it decreased in the Geloplasma group (−22 (−47 to 16 [−84 to 113])%), p = 0.02 between groups. The shorter reperfusion times and increased reperfusion rate suggest that Volulyte maintains better microvascular reactivity than Geloplasma.