The chemical reactions of aquatic Pu(IV) in dilute concentrations are of cardinal relevance for the appraisal of its thermodynamic solubility. Spontaneous colloid formation and disproportionation reactions hinder a straightforward approach and lead to a large number of controversial results in the past. The present work describes a combination of several complementary methods to investigate the solubility of Pu(IV) as a function of pH. At first, laser-induced breakdown detection (LIBD) was applied for colloid quantification. Its sensitivity is higher than photon correlation spectroscopy (PCS) by 7 orders of magnitude (for colloids smaller than 50 nm) and applicable down to 5 nm-particles. To obtain optical absorption spectra for speciation a new diode array spectrometer with a 1m capillary cell (550 μm in diameter), which is 100 times more sensitive than a normal spectrometer, was applied. Its detection limit for Pu(III) and Pu(IV) is below μM. Additionally the capillary electrophoresis (CE) coupled with ICP-MS was used for speciation. (orig.)