Tetravalent actinides (Th, U, Np, Pu) have a strong tendency to hydrolysis, polymerization and colloid formation even in acidic solutions. To quantify and characterize the formed actinide species is of utmost interest for the mechanistic understanding of chemical behaviors of actinides in natural waters. Here we present the spectroscopic study on interactions between Cm/Eu and Th in the course of coprecipitation, employing a novel combination of different laser spectroscopic approaches. Speciation is studied by time-resolved laser fluorescence spectroscopy (TRLFS), and the colloid formation by laser-induced breakdown detection (LIBD). (orig.)