CHLORINE IS UBIQUITOUS ON THE MARTIAN SURFACE OCCURRING BOTH AS CHLORIDE AND PERCHLORATE. CL IS EXTREMELY HYDROPHILIC AND TRACKS CLOSELY WITH WATER. CHLORINE CONCENTRATIONS IN DIFFERENT RESERVOIRS OF MARS THEREFORE PROVIDE INFORMATION REGARDING THE VOLATILE CONTENTS OF AND MIGRATION BETWEEN DIFFERENT MARTIAN RESERVOIRS THROUGH TIME. IN ADDITION TO MEASURING CL CONTENTS WE CAN ADD CL ISOTOPE RATIOS THEREBY PROVIDING FAR MORE INFORMATION THAN WOULD BE OTHERWISE AVAILABLE FROM THE CONCENTRATION DATA ALONE. THE ISOTOPE SYSTEMATIC OF CHLORINE ARE UNIQUE AND PROVIDE INFORMATION THAT IS NOT AVAILABLE FROM OTHER ISOTOPIC SYSTEMS. 1) CHLORINE ISOTOPE FRACTIONATION AT HIGH TEMPERATURES IS NEGLIGIBLE. IT THEREFORE CARRIES A RECORD OF THE DELTA-37CL VALUE THROUGHOUT IGNEOUS PROCESSING; 2)THE DELTA-37CL VALUE OF EARTH AND PRIMITIVE CHONDRITES ARE BOTH CLOSE TO 0%. ANY DIFFERENCES FROM THIS VALUE FOR THE PRIMITIVE MANTLE RESERVOIR OF MARS INDICATE A DIFFERENT CL SOURCE DURING PLANETARY FORMATION; 3) APPRECIABLE CL ISOTOPE FRACTIONATION CAN OCCUR AT LOW TEMPERATURES SO THAT THE ISOTOPIC COMPOSITION OF CRUSTALLY CONTAMINATED SAMPLES ARE EXPECTED TO BE DISTINCT FROM THE MANTLE; 4) CL CONCENTRATION IS HIGH IN THE CRUST LOW IN THE MANTLE. AS A RESULT EVEN MINOR CL CONTAMINATION BY CRUSTAL MATERIAL SHOULD DRAMATICALLY CHANGE THE DELTA-37CL VALUE OF A ROCK.