The performance characteristics of the capacitive slot, or a slot placed between the feed and ground connections, in a planar inverted-F antenna (PIFA) are comprehensively analyzed. The PIFA capacitive slot behavior is measured inside a two antenna system within a mobile phone where the first antenna is a multiple band PIFA and the second antenna is a higher frequency band PIFA directly overlapping with the first antenna higher frequency band. The dual band PIFA in this paper is designed to be resonant in the quad-band GSM+3G/4G, and the second PIFA is resonant in the 3G/4G frequency bands. The capacitive slot has three types of behaviors: affect the matching of existing frequency resonances, induce another frequency resonance, and improve the isolation between the two antennas. Together with optimal antenna ground and feed placement, the capacitive slot can act as a notched bandstop filter to decrease the S21 mutual coupling between the two antennas by over 20 dB and decrease the envelope correlation by almost one order of magnitude.