Hard modelling involves the fitting of a defined mathematical relationship to measured data. This traditionally requires measurements that are done under ideal conditions, such as thermostated solutions, constant ionic strength, etc, rn industrial situations, such conditions are often not maintained and recent attempts have been undertaken to allow the analysis of such data. In this contribution, we report on the hard modelling analyses of reactions with non-constant catalyst concentration. The reversible acid-catalysed esterification reaction of cyanoacetic acid with ethanol was investigated. (C) 1999 Elsevier Science B.V. All rights reserved.