In studies on internal CO2 transport, average xylem sap pH (pHx) is one of the factors used for calculation of the concentration of dissolved inorganic carbon in the xylem sap (). Lack of detailed pHx measurements at high temporal resolution could be a potential source of error when evaluating  dynamics. In this experiment, we performed continuous measurements of CO2 concentration ([CO2]) and stem temperature (Tstem), complemented with pHx measurements at 30-min intervals during the day at various stages of the growing season (Day of the Year (DOY): 86 (late winter), 128 (mid-spring) and 155 (early summer)) on a plum tree (Prunus domestica L. cv. Reine Claude d'Oullins). We used the recorded pHx to calculate  based on Tstem and the corresponding measured [CO2]. No statistically significant difference was found between mean  calculated with instantaneous pHx and daily average pHx. However, using an average pHx value from a different part of the growing season than the measurements of [CO2] and Tstem to estimate  led to a statistically significant error. The error varied between 3.25 ± 0.01% under-estimation and 3.97 ± 0.01% over-estimation, relative to the true  data. Measured pHx did not show a significant daily variation, unlike [CO2], which increased during the day and declined at night. As the growing season progressed, daily average [CO2] (3.4%, 5.3%, 7.4%) increased and average pHx (5.43, 5.29, 5.20) decreased. Increase in [CO2] will increase its solubility in xylem sap according to Henry's law, and the dissociation of  will negatively affect pHx. Our results are the first quantifying the error in  due to the interaction between [CO2] and pHx on a seasonal time scale. We found significant changes in pHx across the growing season, but overall the effect on the calculation of  remained within an error range of 4%. However, it is possible that the error could be more substantial for other tree species, particularly if pHx is in the more sensitive range (pHx > 6.5).