The addition of a small amount of one gas to another changes the viscosity. So the holdup of a suitable tracer gas in a carrier gas flowing in a packed bed is directly related to the pressure drop. An apparatus is described to make this measurement. By following the change in tracer holdup during the course of a purging experiment, it is possible to determine the low-order moments of the residence-time distribution. The theory and experimental design were tested using argon as tracer and helium as carrier gas. The results were consistent with those obtained by other methods. This experimental method is not subject to the tl weighting problem that hinders the use of the impulse response method of measuring the second moment of the residence-time distribution.