As part of a detailed study of the nomenclature and taxonomy of the New Zealand species of Pseudocyphellaria it has been noted that the chemistry of the P. impressa (Hook & Taylor) Vainio aggregate, that is species with a markedly faveolate (honeycomb- like) upper surface and a green algal phycobiont, is particularly complex. The 45 lichen substances detected fall into four distinct groupings, here designated as 'Codings'. The aim of this contribution is to investigate suitable methods for revealing this chemical assemblage by t.l.c. and, as far as is currently possible, to identify the range of substances. No conscious attempt is made to assign the four chemical Codings to particular taxa or species groupings; the taxonomy of the group will be discussed elsewhere. The four Codings proved to be remarkably uniform and constant considering the range and number of substances involved; such variation that occurred was only in the presence and concentration of accessory substances. Although this work only concerns New Zealand material, similar patterns occur in related taxa of the group present in southern South America, south-east Australia and Tasmania.