Aqueous solubilities of nonylphenol (NP), octylphenol (OP) and their ethylene oxide adducts with 1–5 ethoxy groups were determined using a generator column technique followed by subsequent off-line HPLC analysis of the aqueous solutions. The solubilities of OPnEO oligomers (8.0–24.5 mg/L) were significantly greater than those of the corresponding NPnEO oligomers (3.02–9.48 mg/IL) indicating a predominant influence of the hydrophobic chain length on APnEO solubility. A linear relationship between solubility and the number of EO groups was established for both oligomer series. However, nonethoxylated alkylphenols showed irregularly enhanced solubilities as compared to the corresponding ethoxylated compounds probably caused by different impacts of phenolic and alcoholic OH functional groups. NP and NP1EO showed a gradual solubility increase towards higher temperatures in the temperature range examined (2–25 °C), while an irregular behaviour was observed for NP2EO. The concentrations of individual oligomers in solutions of APnEO oligomer mixtures were below their saturation levels, while the oligomer composition in the solution showed significant change compared with the original one, reflecting different solubilities of the individual oligomers as pure substances.