The areal detection efficiency of a channel electron multiplier array (CEMA) and the nature of its secondary emission surface were investigated with the ion microprobe mass analyzer (IMMA). Previous speculations of detection efficiency based on simple geometric arguments of channel‐to‐interchannel area ratios are shown to be in error. With a positive bias applied to a front electrode surface of high secondary yield, areal detection efficiency of a CEMA may approach 100% even for single‐ion impacts. The secondary emission properties of a CEMA are shown to be determined by an alkali‐rich surface layer on each channel wall, and not a concentration of Pb or PbO as previously thought. Both reversible and irreversible gain degradation observed when heating a CEMA in vacuum are interpreted as a change in the composition of this layer.