An experiment was designed to determine whether the singleton and geminate consonant categories overlap at fast speaking rates in Persian. It was found that both geminate and singleton stop durations decrease when spoken at faster rates. The geminate durations decrease more than the singleton durations, but the categories remain distinct at each speaking rate. The article discusses whether the threshold value for the category distinction is adjusted according to the speaking rate. The ratio of the stop duration to the preceding vowel duration is not a reliable invariant cue for gemination in Persian, since the preceding vowel lengthens before a geminate consonant. A parameter k representing a proportion of the average syllable duration is proposed as an invariant discriminator between the categories. This parameter is useful as a discriminator only if a minimum stop duration is assumed.