This article considers a Q-band cylindrical TE011 electron paramagnetic resonance cavity with slots cut parallel to wall currents to allow penetration of magnetic field modulation and with mode suppression gaps in the end regions of zero wall currents to reduce interference from nearby modes. Origins of the small amount of rf leakage generally observed from modulation slots have been studied using a finite element computer program and ways to eliminate this leakage have been found. The leakage is caused by field distortions from the waveguide coupling iris, which is centered on the cylindrical sidewall. The distortion is of two distinct types, which result in comparable leakage levels. One is from iris fields that have components perpendicular to the fields of the mode of interest. The iris near fields couple to a nonresonant circumferential radial mode in the modulation slots, like that of a sectorial waveguide horn. The other type of rf leakage is caused by coupling of the aperture to nearby cavity modes that leak from the modulation slots if they have currents across the slots. End gaps have been used to “suppress” the TM modes that, without the gaps, are degenerate with the TE011. We find that the TM modes are not suppressed, but rather altered, as are most modes. The modes are combinations of coaxial modes that are excited in the end-gap regions coupled to normal cavity modes. The most desirable way to reduce leakage is the combination of a thin slotted iris, thin modulation slots and sufficiently thick cavity walls. Electric discharge machining techniques can be used to produce slots of the required dimensions. A less desirable way to reduce the rf leakage is to place a floating conducting shield just outside the modulation slots. If resistive material is placed in the end gaps, rf leakage from the modulation slots decreases because the amount of energy coupled to an adjacent cavity mode is reduced, which increases the Q of the TE011 mode.