For on-chip data communication with galvanic isolation, a monolithically integrated optocoupler is strongly desired. For this purpose, silicon (Si) avalanche mode LEDs (AMLEDs) offer a great potential. However such AMLEDs have a relatively low internal quantum efficiency (IQE) and high power consumption. For the first time, in this work, data communication in a monolithically integrated optocoupler is experimentally demonstrated. The novelty of this work is the use of highly sensitive single-photon avalanche diodes (SPADs) for photo-detection to compensate for the low IQE of AMLEDs. We investigated our optocoupler realized in a standard 140 nm CMOS SOI technology, without post-processing, for various LED designs and points of operation. The power consumption of the AMLEDs is minimized through a novel AMLED design and employment of a low power LED driver circuit. The advantages of AMLEDs over forward biased Si LEDs are also demonstrated. For the best AMLED design, the achievable data rate is few Mbps and the energy consumption a few nJ/bit. The active area of the proposed systems is < 0.01 mm(2).