SummaryThree 0.01mFe(ClO4)3Solutions ofR= 0, 1, and 2, respectively, were studied over a period of 8 months, whereRis referred to the HCO3−/Fe3+mole ratio in preparation. TheR= 0 solution was initially light yellow and clear but rapidly changed to a dense cloudy Suspension after a 9‐day induction period. Hydrolysed species rapidly grew from650 nm. Particles of intermediate size were not observed during the entire period of study. Settleable FeOOH precipi‐tate was first observed in 5 weeks with 23 per cent of its iron noted as precipitate at the end of 5 months. TheR= 1 andR= 2 Solutions were reddish‐brown of low turbidity. Rapid hydrolysis and polymerization were observed immediately after preparation. The hydrolysed species gradually grew in size during ageing and no settleable FeOOH precipitate was observed for at least two years. A hypo‐thesis based on Lamb and Jacques's model of Fe3+hydrolysis is proposed to interpret the reactions taking place in these three Solutions. In theR= 0 solution, the initial number of nuclei for condensation is low, whereas the poten‐tial supply of Fe(OH)3is abundant. In theR= 1 andR= 2 Solutions the number of nuclei initially formed is high, whereas a limited amount of mono‐meric Fe(OH)3is available for condensation. The initial number of nuclei relative to the concentration of monomeric species is the key factor that governs the appearance and properties of th