Inorganic arsenic (iAs) is an established human carcinogen. Although methylation of iAs was considered as a detoxification mechanism, recently it is considered as an intoxification pathway in mammals. Our study population consisted of four groups A-D with drinking water iAs concentrations 33 ± 7, 148 ± 34, 210 ± 2.6, and 248 ± 59 μg As/l (mean ± SE), respectively in West Bengal, India. The ratios (monomethylated arsenicals)/(inorganic As metabolites - arsenate) = (MMA + DMA)/(iAs Met - iAsV), (dimethylated As)/(mono- and dimethylated As) = (DMA)/(MMA + DMA), and (dimethylated As)/(inorganic As metabolites - arsenate) = (DMA)/(iAs Met - iAsV) were used to assess methylation efficiency in the present study. High performance liquid chromatography-inductively coupled argon plasma mass spectrometry (HPLC–ICP MS) was used to determine As species in spot urine samples. The detection limit of As compounds was 0.14-0.33 mg As/l. All trivalent arsenicals were stable for up to 2 months when arsenic spiked urine samples were stored at –28o C without any preservatives. Although females appeared to be better methylators than males and children (considering first and total methylations), they were statistically not significant (p>0.05). Only second methylation capacity was statistically significant between different age groups (p<0.05). Although methylation capacity is not statistically conclusive in the present study, this is the first study, which documents the results on reduction capacity of the As-affected population in the endemic areas. Another outcome of this study suggests that researchers must consider the intake route of As via food-chain during the preparation of toxicity model of As.