Silver-colloid solutions have been used as the medium for surface-enhanced Raman spectroscopy (SERS) for over two decades. UV-Vis absorption is often used to determine the state of aggregation of the colloid, considered a critical parameter for a successful SERS experiment, especially when using an IR excitation source such as an Nd:YAG laser. We present results from parallel studies of silver colloid aggregation and UV-Vis absorption showing that a significant change in the aggregation state of the colloid does not necessarily translate into a better SERS substrate when using an IR excitation source. Conversely, we show that a small addition of chloride ions sufficient for inducing FT-IR SERS does not produce any detectable colloid particle aggregation. These results may give new insight into how to optimize experimental conditions for SERS when using a near-IR excitation source. Copyright © 2005 John Wiley & Sons, Ltd.