We present low-angle light scattering data on aggregation runs of dens
e colloidal systems. We show that the scattered pattern exhibits a pea
k at finite wavevectors if the reaction occurs in a diffusion limited
regime, and universal scaling similar to that of spinodal decompositio
n applies. If the runs are made under conditions closer to reaction li
mited behaviour, however, the peak appearance is delayed until it is e
ventually lost altogether.