This paper presents an impregnation technique based on fluorescent ethanol that makes it possible to do plane section and thin section analysis on concretes of very low w/b-ratios. The technique is called Fluorescent Liquid Replacement technique (FLR) and is based on a principle where the pore water is slowly replaced by ethanol. Tests have been done on virgin concrete samples and in combination with elements of traditional preparation procedure, drying and evacuation. A comparison to epoxy impregnation is done. Plane sections and thin sections are made from five different concretes with w/b-ratios 0.20, 0.25, 0.30 and 0.40 containing 10 8 silica fume and w/b-ratio 0.35 without silica fume. Results show that the FLR technique gives impregnation depths that are several orders of magnitude larger than what is obtained with traditional procedures. Measured fluorescence intensities show that w/b-ratios may be determined in the range from 0.20 to at least 0.40. An extensive crack pattern was seen at lower w/b-ratios using the traditional epoxy impregnation technique and FLR in combination with drying and evacuation which is in agreement with earlier experiences. FLR technique used on wet virgin samples did not produce cracks. The observed cracks seem to be an effect of preparation. Impregation measurements are supported by exchange studies done by gravimetry and Nuclear Magnetic Resonance technique on samples of hydrated cement pastes 1063