We present ion mobility measurements for small gold cluster cations, Au-n(+)(nless than or equal to10) in the temperature range between 77 and 300 K. While at room temperature all cluster sizes show only one sharp peak in the arrival time distribution, the Au-9(+) signal becomes bimodal at temperatures below 140 K. We interpret this unusual behavior in terms of two different isomeric forms of this ion which undergo temperature dependent interconversion reactions. From the reaction rate the activation barrier for this isomerization can be estimated to be 10-20 kJ/mol.