The frequency pulling of an oscillating magnetron is measured by inserting a reactor in the output feeder, which produces a reflection of constant magnitude but variable in phase; the frequency pulling is usually estimated on the assumption of constant v.s.w.r. in the feeder. In practice, when multiple reflected waves exist in the feeder the v.s.w.r. is not constant, but it is found that the amplitude/phase variation of the resultant reflected wave is such that the locus of the voltage reflection coefficient is a circle in the Smith chart. An idealized equivalent circuit for the magnetron is assumed and the theoretical frequency pulling is estimated. The performance of a typical waveguide system used in magnetron testing is examined with regard to the possible variation in measured frequency pulling.