The influence of the end groups on the glass transition temperature, T-g, of the polyisoprene block of styrene-isoprene diblocks was studied by differential scanning calorimetry. The end-functionalized styrene-isoprene diblock samples used have dimethylamino and sulfobetaine groups attached on the polyisoprene chain end. An increase in the low temperature (polyisoprene-type) T-g value was observed for the lower molecular weight zwitterion capped polymers. This effect is attributed to restricted segmental mobility of the polyisoprene chains found in the proximity of the ionic aggregates.