Noise measurements were performed on several commercially available optical isolators in order to examine the signal detection limits of typical devices. It was found that, in general, optical isolators are very noisy devices exhibiting all of the common types of noise usually found in bipolar junction transistors. A large number of devices exhibited burst noise which dominated their low-frequency noise performance. Experimental data indicates that the source of burst noise is the phototransistor detector. A comparison of experimental data to existing burst noise theory is presented.