Viscosities of seven different epoxy-type resins currently used in different laboratories to impregnate cement paste and concrete specimens for backscatter-mode Scanning electron microscopy were measured. The measured viscosities were found to range over more than two orders of magnitude. The most viscous of the resins tested (Araldite AY-103) apparently has difficulty in achieving the necessary penetration into dense concrete specimens. Many of the resins thicken spontaneously and gel relatively quickly after mixing, which limits their penetration, but satisfactory results can be obtained with care. Two of the resins (LR White and Spurr’s) are of much lower viscosity than the others and do not thicken spontaneously. This permits use of extended penetration times and deep penetration into even very dense concrete specimens.