A custom-developed colorimetric device is presented for arsenic ion detection in drinking water. The device utilizes standard reagents, which are used in arsenic field-testing kits, in its custom microfluidic system. The response for arsenic concentrations is linear in the 5-20μg/l (ppb) range, thus our device is capable to distinguish drinkable water based on the World Health Organization (WHO) guidelines1 limit of 10 μg/l. The effect of elevated water sample temperature on the kinetics and performance of the device is investigated in order to reduce the required time for the measurements. Stable and reliable arsenic concentration values were obtained after 25 min at 50 oC water sample temperature.