  In the volume fraction range (0.005,0.08), we have obtained the temporal
evolution of the structure factor $S(q)$, in extensive numerical simulations of
both diffusion-limited and reaction-limited colloid aggregation in three
dimensions. We report the observation of scaling of this structure function in
the diffusion-limited case, analogous to a spinodal decomposition type of
scaling. By comparing $S(q)$ with the pair correlation function between
particles, we were able to identify the peak in the structure factor as arising
from the correlations between particles belonging to nearest-neighbor clusters.
The exponents $a\prime$ and $a\prime \prime$ that relate the position and the
height of the maximum in $S(q)$ vs. time, respectively, were also obtained and
shown to differ somewhat from the spinodal decomposition exponents. We also
found a terminal shape for $S(q)$ that corresponds to a close packing of the
clusters after gelation. Moreover, this picture was shown to be valid in a
concentration range larger than the one suggested in recent experiments.
Although the $S(q)$ for reaction-limited colloid aggregation does not show a
pronounced peak for the earlier times, eventually the peak stretches and
becomes higher than in the diffusion-limited case. The $S(q)$ curves, however,
do not present the scaling shown for diffusion-limited aggregation.
