Aluminium solubility and toxicity increase with acidification. There is no standardized analytical method for the determination of inorganic monomeric Al (Al_im), which is the form that causes toxicity to fish. Separation by cation exchange is commonly combined with other analytical methods, such as complexation with pyrochatechol violet (PCV) or 8-hydroxyquinoline (HQ) and total quantification using graphite furnace or inductively-coupled plasma emission. Data from 14 laboratories were obtained for a dilution series of Al_im samples; the results of the Al_im analysis were statistically evaluated. The Al_im levels were altered through pH, which was controlled by the addition of calcium hydroxide. Confounding parameters such as total organic carbon (TOC) or fluoride (F) were controlled. The total determination and HQ methods yielded significantly higher Al_im concentrations than the PCV method. Pretreatment by passage through a 0.45 µm filter and pH-adjustment of the ion exchange column had no apparent effect on the Al_im yield. However, ultra filtration (<10 kDa) caused a significant reduction in the Al_im concentration using the HQ method. The ultra filtrated Al_im fraction was similar to the PCV results in the interlaboratory comparison. Retention of colloidal bound Al in the cation exchange column may result in overestimation of Al_im when the total and HQ methods are used. Estimated Al_im concentrations derived from two equilibrium models were similar to PCV-derived Al_im concentrations, as well as the HQ method using ultra filtrated water. The fact that the PCV method does not detect colloidal Al, neither before nor after ion exchange, makes this a preferred technique for Al_im analysis. Because of the variability in the reported Al_im concentrations that can arise when different analytical procedures are used, the adoption of a single, reliable technique will facilitate inter-study comparisons and provide consistency in the detection of trends in environmental monitoring programs.