This study examines effects of solution chemistry on the straining of colloids in porous media under unfavorable conditions. Saturated column experiments were conducted with various sizes of latex particles and glass beads at different ionic strengths. A two- or three-step procedure was used for colloid attachment and release, following dissection of the columns to determine distribution profiles of retained colloids. The effluent breakthrough curves and spatial distributions of the retained colloids in the columns were modeled with a convective diffusion equation including terms of attachment and straining. Results show that (i) straining increases with the decrease of glass beads diameter (d_c) and with the increase of colloid diameter (d_p); (ii) fitted straining coefficient increases with the increase of attachment rate (or ionic strength); (iii) the threshold of d_p/d_c (above which straining occurs) determined using glass beads as porous media is much larger than those determined by sand grains. Our study indicates that a deeper secondary minimum depth facilitates colloids wedged and retained in straining sites, and the irregular shapes of natural porous media (e.g. sand grains or soil) play critical roles in the straining (i.e., grain-to-grain contact) of colloids when d_p/d_c is small.