Radio-Isotope cone penetrometers (RI-cones) are developed to measure the basic soil properties like water content and soil density under in situ conditions in real time along with other parameters such as soil strength and hydrogeologic conditions of subsurface soils. In this paper, a brief description of these cone penetrometers is followed by their calibrations and the use of single calibration curve for different types of soils is discussed. Two case studies have been presented. It has been shown that under marine conditions a correction is required for the measured water content as the presence of chloride ions, a very strong neutron absorber, can underestimate the measured water content.