The aim of the study was to validate a semiquantitative analytical method to identify the aluminium(III) [Al(III)] concentration in 99Mo/99mTc generator eluates to check the European Pharmacopoeia (Ph. Eur.) requirement (<5 μg/ml). Three different solutions measuring 20 ml – 0.2% 1,10-phenanthroline, 0.05% chrome azurol S and 20% hexamethylenetetramine – were prepared. A cellulose filter paper was subsequently immersed in them, dried overnight at room temperature and cut into rectangles. A volume of 5 μl of first eluates of various 99Mo/99mTc generators was placed onto a reagent paper and the spot colour was compared with a standard aluminium solutions scale (0–100 μg/ml). A cyan/magenta/yellow/key (CMYK) model analysis was adapted to quantify the intensity of colour on the paper, and the presence of aluminium in the eluates was detected by a spectrophotometer. Small changes in standard solution pH (4.1–5.2) and chrome azurol S concentration did not affect the analysis. The cyan channel image analysis was proportional to the Al3+ solution concentration (y=25 019x+1489, R2=0.9554 within 2.5–8 μg/ml). The detection limit for aluminium by the visual test method is about 1 μg/ml, and fading is absent. The cyan channel image analysis method is independent of the observer and is applicable for the evaluation of the chemical purity of 99Mo/99mTc generator eluates. Our colorimetric ‘spot test’ is advantageous for the visual evaluation of Al3+ pertechnetate concentrations as required by Ph. Eur. showing a sensitivity and a limit of detection superior to that of commercially available spot systems.