Number of independent partial structure factors for a disordered n-component system

n(n + 1)/2 independent diffraction measurements are needed to obtain comple te partial structure information for a disordered n-component condensed sys tem. A previous assertion that only n of these are independent is shown to be inapplicable, for different reasons, to both the three-dimensional parti al structure factors S-ab(Q) and the orientational averages S-ab(Q). The fl uctuations of S-ab(Q) about S-ab(Q) and the effects of thermal motion are d iscussed. [S0163-1829(99)00491-4].