ESD protection capability of SOI CMOS output buffers has been studied with Human Body Model (HBM) stresses of both positive and negative polarity. Experimental results show that the ESD discharge current is absorbed by the NMOSFET alone. Unlike bulk technologies where the bi-directional ESD failure voltages are limited by positive polarity stresses, SOI circuits display more serious reliability problem in handling negative ESD discharge current. Bulk NMOS output buffers fabricated on the substrate of the same SOI wafers, after etching away the buried oxide, have been used to compare the ESD protection capability between bulk and SOI technologies. The ESD voltage sustained by these `bulk' NMOS buffers is about twice the voltage sustained by conventional SOI NMOS buffers. This scheme is proposed as an alternative ESD protection for SOI circuits. The effectiveness of ESD resistant design strategies developed in bulk-substrate technologies when transferred to SOI circuits is also discussed in this paper.