The present work investigates the formation of well-defined heteroaggregates from a binary mixture of a red and a yellow azo-dyestuff in the presence of Mg2+ ions. Combined static and dynamic light scattering together with laser induced liquid bead ion desorption mass spectrometry (LILBID-MS) has been applied to characterize the states of the pure red dye and the pure yellow dye as well as of their mixture in aqueous solution without Mg2+. These experiments indicated that a structural reorganization on a molecular scale takes place as soon as the two dyes are combined. Solutions of the combined red and yellow dye contain micelle-like mixed entities with a size of a few tenths of milometers. Upon the addition of Mg2+, these micelles vanish in favor of elongated heteroaggregates, which grow by a stepwise addition of smaller building units. As unraveled by UV/vis spectroscopy, the heteroaggregates that are formed from the red and yellow azo-dye in the presence of Mg2+ obey a stoichiometric ratio of the two components of 1:1. A new multiangle scattering instrument allowed us for the first time to follow this aggregation process at the stoichiometric ratio by time-resolved combined static and dynamic light scattering, thereby providing further aspects of the worm-like nature of the growing heteroaggregates.