Probable causes for the deterioration of the physical properties of paper as a result of repeated handling have been determined. A flexing test has been devised that evaluates the relative durability of paper. Investigations with the scanning electron microscope indicate that the fibrillar component of paper deteriorates during flexing and is probably responsible for the decline in stiffness as well as the modulus of paper. Because of the ever-increasing use of automatic document handling equipment, stiffness retention with handling is essential, since limp documents are difficult, if not impossible, to process automatically. Unfortunately, stiffness declines rapidly as paper is flexed or handled. The rates of deterioration for all other physical properties of paper during flexing are independent of each other and vary from one paper to another. The rate at which certain properties deteriorate is independent of the quality of the paper. No correlation has been found between any paper property and durability.