Acid absorption is a routine analytical method used in the manufacturing of lead oxide, that is, then further used to manufacture Pb-acid battery electrodes. This study had a new look at the definition of acid absorption being an indication of the acid reactivity of the oxide forming certain lead sulphate-related phases. Quantitative powder X-ray diffraction analysis of the acid reaction products showed there were significant differences in the phases formed that could be related to the amount of acid used in the acid absorption test. The study also showed that there were significant changes in the surface area of the oxides once they had reacted with the acid, where a traditionally slow reacting oxide such as β-PbO would show the greatest increase in material surface area once reacted with the acid. The accuracy of the method used by various laboratories was also studied by comparing the results obtained from two different methods and from three different laboratories. The results showed that there were significant differences between the reported values, and that one should with caution compare acid absorption numbers obtained from different laboratories.