A disposable ion-selective optode for mercury based on trityl-picolinamide (T-Pico) as neutral ionophore was developed. The sensing layer consist of plasticised PVC incorporating T-Pico as a selective ionophore for Hg2+, ETH 5418 as a chromoionophore, and potassium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate as lipophilic salt. The measurement principle is based on an ion-exchange mechanism. When the optode membrane is introduced into a water sample for 5 min, there is a colour change from red to blue, depending on the mercury concentration (pH 4.7), making it possible to use the absorbance at 665 nm as the analytical signal. The optode membrane response to Hg2+ was not fully reversible; however, it reveals a very good selectivity to many cations including alkali, alkaline earth, transition, and heavy metal ions. The detection limit for Hg2+ is 5.0x10(-7) M at pH 4.7. The response characteristics of the sensor including dynamic range, reproducibility, response time, and lifetime are discussed in detail. This sensor was used for the determination of mercury in environmental water samples with satisfactory recovery.