This paper introduces an error analysis of the international standard procedure for the measurement of optical absorption, ISO 11551. In this paper expression are developed for the uncertainty in a given absorptance measurement. The expressions for the uncertainty are developed using a standard propagation of error technique for both the pulse and gradient method. The paper includes uncertainty calculations based on typical experimental parameter values and their associated uncertainties for both methods. It is shown that measurements made by the pulse method have smaller uncertainty than gradient method measurements.