Introduction The mobile-phone-based Bedfont iCO TM Smokerlyzer ® is of unknown validity and reproducibility compared to the widely-used piCO + Smokerlyzer ® . We aimed to compare the validity and reproducibility of the iCO TM Smokerlyzer ® with the piCO + Smokerlyzer ® among patients reducing or quitting tobacco smoking. Material and Methods Methadone-maintained therapy (MMT) users from three centers in Malaysia had their exhaled carbon monoxide (eCO) levels recorded via the piCO + and iCO TM Smokerlyzers ® , their nicotine dependence assessed with the Malay version of the Fagerström Test for Nicotine Dependence (FTND-M), and daily tobacco intake measured via the Opiate Treatment Index (OTI) Tobacco Q-score. Pearson partial correlations were used to compare the eCO results of both devices, as well as the corresponding FTND-M scores. Results Among the 146 participants (mean age 47.9 years, 92.5% male, and 73.3% Malay ethnic group) most (55.5%) were moderate smokers (6–19 cigarettes/ day). Mean eCO categories were significantly correlated between both devices (r=0.861, p