In an 8-12 mm PS-3-INS catalyst thoroughly passivated and discharged from the reactor, structural promoters (At, Ca, Mg, Si) were unaffected, S was present in the top layer, and K losses were 26% in the top and up to 10% in other layers. In mid-layers, Fe crystallite size growth was max. Reactor's operating lines were calcd. by assuming various catalyst activity variation models, plug flow of the gas through the bed, and the exptl. kinetic characteristics of the catalyst Most affected were temp. profiles; ammonia production only in 2%. The catalyst activity drop, only 30% in a 12-year-long operation of a 500-t NH3/day plant, was attributed to catalyst's thermal stability and high purity of the synthesis gas. The coalescence filters installed downstream of fresh gas 171 compressors reduced the oil content in the product to 3 ppm.