  A recent Letter [Phys. Rev. Lett. 103, 156101 (2009)] reports the
experimental observation of aggregation of colloidal particles dispersed in a
liquid mixture of heavy water and 3-methylpyridine. The experimental data are
interpreted in terms of a model which accounts solely for the competing effects
of the interparticle electrostatic repulsion and of the attractive critical
Casimir force. Here we show, however, that the reported aggregation actually
occurs within ranges of values of the correlation length and of the Debye
screening length ruled out by the proposed model and that a significant part of
the experimental data presented in the Letter cannot be consistently
interpreted in terms of such a model.
