Where the invention concerns improving a process to manufacture a known chemical compound, then the closest prior art is confined to documents describing that compound and its manufacture. Comparison with these alone shows whether an improvement has been achieved which can thus be taken into account in formulating the problem the invention sought to solve (T 641/89, T 961/96, T 713/97, T 948/01, T 833/02, T 339/03). In the case of inventions concerning a special process for use with a particular chemical substance having necessarily specific characteristics, determining the closest prior art must involve, above all, considering only those documents which describe a generically corresponding process for using precisely this particular chemical substance with its specific characteristics (T 1285/01, T 354/03, T 1652/08). This accurately and objectively reflects the actual situation in which the skilled person found himself on the priority date of the contested patent (T 793/97).

The above considerations regarding the closest prior art also apply to production processes for subject-matter other than a chemical compound. In T 325/97 the patent related to a method for manufacturing a device for controlled delivery of nicotine from an adhesive reservoir. In T 373/94 the board also applied the principles and conclusions laid down in T 641/89 where the invention related to the improvement of a manufacturing process for prefilled plastic syringes.