This work is concerned with the extraction, at the laboratory level, of uranium and thorium from Swiss granites. The Mittagfluh, Bergell and Rotondo granites and the Giuv syenite offered a wide range of U and Th concentrations: 7.7-20.0 ppm U and 25.5-67.0 ppm Th. U and Th were determined in leachates by the fission track method and by spectrophotometry respectively. Samples, containing < 0.3 μg U and 4 μg Th, could be measured with an accuracy of 10% for U and 5% for Th. Leaching experiments were performed during which the following parameters were varied: granite type, grain size, acid type, acid concentration, temperature and time. There were large leaching differences between the granites studied. Grain size caused no great variation; the extracted percentages of U and Th were practically the same for any particular rock with either 12% (coarse) or 71% (fine) of the grain size finer than 0.125 mm. The leached fraction with H2SO4 was somewhat smaller than with HCl or HNO3. The difference in leaching with 2, 5 and 10 M HCl was negligible. Temperature was the most important parameter. Sharp differences in extraction occurred between 20°, 50° and 80°C. At 80°C more than 85% of the U and Th were extracted. The extraction curve (percent extracted as a function of time) of aliquots sampled after 1, 2, 4, 8, 12 and 24 hr. showed a plateau after 8 hr. As a general rule, Th was more easily extracted than U. The granites investigated showed widely different U and Th leachability (13-75% U, 32-75% Th upon treatment of rock powder in 5 M HCl at 20°C for 24 hr.). A separate study was devoted to relate these differences to peculiarities in the U and Th microdistribution in these rocks (Part 2).