Increasingly potent formulations of concentrated cannabis products containing high levels of Δ 9-tetrahydrocannabinol (THC) are being consumed along with alcohol. This combination is known to acutely impair cognitive and psychomotor abilities, although few if any studies have evaluated alcohol and cannabis co-use using neural measures. This feasibility study explored the acute effects of co-intoxication on behavioral performance and brain activity, measured via electroencephalography (EEG), during a performance monitoring task administered three times over a 4.5-hour period. Participants (N=17, 65% male) were heavy-drinking community members (age M=31.7 years; SD=10.27) who also regularly use cannabis. EEG recordings were made while participants completed a speeded visual-flanker task at baseline (sober) before consuming a standardized dose of alcohol and a cannabis concentrate product, followed by EEG recordings 1-hour (intoxicated) and 4-hours post co-consumption (recovery). Recordings were processed to obtain six event-related potential components: stimulus-locked (N1, P2, N2, P3) and response-locked, error-related negativity (ERN) and error positivity (Pe). Repeated Measures ANOVAs revealed that behavioral measures at the intoxicated timepoint showed an increase in errors (p <.005; η 2 = .35) and significant decrease in response time (RT) (p<.001; η 2 p = .50) compared to sober and recovery periods. Measures of brain activity also demonstrated significant change across time with largest amplitudes when sober, smaller amplitudes when intoxicated, then an increase in amplitude to near sober levels at recovery for stimulus-locked components (P2, N2, P3) and response-locked components (ERN and Pe). Thus, compared to sober and recovery periods, the behavior at the intoxicated timepoint revealed significantly greater impulsivity (RT) leading to significantly higher error rates while measures of cognitive processing (i.e., decision-making, error detection, performance monitoring/adaptation) showed significantly diminished levels of brain activity. Performance and neurophysiology measures remained diminished at recovery compared to baseline. Understanding neurocognitive functioning during co-intoxication may provide insights into specific cognitive processes that may impact function in everyday activities such as driving.