Three experiments (N= 68), using Wynn's procedure, tested 5-month-old infants' looking time reactions to correct and incorrect results of simple addition and subtraction transformations. The aim was to investigate both the robustness and the parameters of infants' arithmetic competence. Experiments 1 and 2 (N= 44) were replications of Wynn's first two experiments in which infants were shown addition (1 + 1 = 1 or 2) and subtraction (2 − 1 = 1 or 2) requiring imprecise calculation. Experiment 3 (N= 24) was a subtraction counterpart (3 − 1 = 1 or 2) to Wynn's third experiment requiring precise calculation of addition (1 + 1 = 2 or 3). Unlike Wynn, we found no systematic evidence of either imprecise or precise adding and subtracting in young infants. Our results, together with the mix of both positive and negative findings from other studies of infant arithmetic, suggest that infants' reactions to displays of adding and subtracting are variable and, therefore, that infants' numerical competencies are not robust. This conclusion is consistent with previous findings indicating that simple adding and subtracting develops gradually and continuously throughout infancy and early childhood.