Laboratory testing of corrosion inhibitors under high temperature high pressure (HTHP) conditions is challenging. HTHP testing has been traditionally performed in closed systems with fixed liquid/gas volume and testing results are usually influenced/compromised by the accumulation of ferrous ions and corrosion products. The aim of the work is to optimize corrosion inhibitor testing conditions at HTHP to generate results of better reliability. The corrosion of carbon steel by CO2 at HTHP was assessed using small working electrodes of large liquid volume-to-sample surface area in autoclaves. The effect of CO2 partial pressure was also investigated. The blank and inhibited corrosion rates were monitored by linear polarization resistance (LPR) and the morphology of coupon surface was measured by vertical scanning interferometry (VSI). The testing results were deemed to be more representative of the field service environment when the amount of ferrous ions and corrosion products was reduced due to the usage of small working electrodes.