A novel ammonia optode was designed using [4-methyl-N,N'-bis(salicylidene)-1,2-phenylenediamine] tributylphosphincobalt(III) perchlorate monohydrate [Co(4-MeSaloph)(PBu)3]+ complex coated on a triacetylcellulose membrane. The sensor could be used at a wavelength of 378 nm to detect ammonia in the range of 8.3 x 10(-4) and -1.25 x 10(-2) M in borate buffer (pH 10). The LOD was found to be 9.5 x 10(-5) M. The response time of the optode was 2-3 min and depended on the concentration of ammonia. This optode was successfully applied for the determination of ammonia in drinking water.