The optimisation of acquisition conditions for EELS spectroscopy of Al x Ga 1Οͺx As heterostructures permits one to find the absolute concentration with a precision of better than D ^0:02 and to detect changes in concentration of ^0.01 for x 0 Οͺ 0:5: In order to achieve this concentration precision, we investigated ways to reduce the influence of three major sources on the inaccuracies of the measurement: the effect of electron channelling which biases the ionisation probabilities on the different atomic sites, the contribution of the sample surface layers and the accuracy in the spectral analysis. An optimal specimen orientation that maximises the stability of the electron densities on Al and As sites without introducing an unacceptable loss of spatial resolution due to sample tilt is found by computing the electron channelling intensity as a function of the specimen tilt angle. The influence of a Ga enriched surface layer on the analysis is demonstrated. Two methods for the extraction of the edge intensity from the spectra are compared. These methods are shown to give the upper and the lower limit of the Ga concentration.