This paper reports on investigations into the way carbon dioxide (CO2) hydrate forms in porous silica gel partially saturated with pure water or with a surfactant solution. The experiments, conducted at two different temperatures (278.2 and 279.2 K) and under a loading pressure of 3.8 MPa, used silica particles of different nominal pore diameters (30 and 100 nm), saturated at 80% pore volume with pure water or with a 100 ppm solution of either sodium dodecyl sulfate (SDS) or polyoxyethylenesorbitan monoleate (Tween-80). They were run following the "hydrate precursor method" developed in previous works (Duchateau et al., 2009, 2010) to form bulk hydrate under controlled subcooling conditions, and adapted for studying hydrate formation behavior in porous media. The work demonstrated that the successive hydrate formation and decomposition cycles involved in this method do not alter the pore size distribution in the porous media. At the two temperatures investigated, silica gel particles with a nominal pore diameter of 100 nm proved better suited to comparing the CO2-hydrate formation behaviors: higher water-to-hydrate conversions (> 90 mol%) were effectively obtained for all the conditions tested making comparison of the results much easier. Of the two surfactants used, only SDS was found to produce a positive effect on both the hydrate formation kinetics and the amount of hydrate formed. Our visual observations of quiescent bulk systems (without porous silica gel) suggest that when SDS is present, CO2 hydrate forms not only at the w/g interface (where it occurs without SDS too), but also in the bulk water phase. This may explain the beneficial effect observed on the porous medium. (c) 2013 Elsevier Ltd. All rights reserved.