A brief historical review has been made of the values assigned to the binding energy of Cls electrons from hydrocarbon contamination on sample surfaces. The particular energy calibration technique used for each electron spectrometer on which the CIS determination was made has been identified. There are notable variations in the CIS electron binding energy for a hydrocarbon contamination layer with respect to the substrate on which it was measured. The possible source and nature of adventitious carbon have been discussed, as have factors which can affect its Cls binding energy value. Sodium phosphate salts have been used to illustrate the relative merits of using an internal energy reference line rather than the Cls electrons from the adventitious carbon layers of these salts. Alternative energy referencing techniques, which include deposition of soot from a candle flame and deliberate 'in situ' condensation of a volatile organic compound onto the sample surface, have been compared with the method under review. Another possible method is the mixing of powdered samples with reference compounds (e.g. graphite). It is concluded that, although the use of Cls electrons from adventitious carbon layers is often a convenient method of energy referencing, interpretation of binding energy data obtained should be treated with caution.

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