We study the formation of silver colloids and the effect of an additive on their growth by measuring the kinetics of silver reduction in the presence of anisic acid, combined with transmission electron microscopy. The kinetics show an initial fast reduction of 1-3% of the total silver present in the solution, then a plateau region with hardly any reduction going on, and finally a region of accelerated growth and aggregation. The results are interpreted quantitatively in terms of the adsorption of the additives on the silver clusters at the early stages of the reaction and a slow-down of their subsequent growth. This is an example for the use of additives to achieve a control over the size distribution of colloids-determining the average size of the particles and the width of the distribution.