The Plessey (M) coal in the Northumberland and Durham coalfields has never had the economic significance of many other coal seams in the region because of its variable thickness and its sporadic occurrence as several thin seams. Where mined, the seam has been long known to display aberrant coking properties compared with coals above and below in the succession. Samples of Plessey coals were analysed from nine offshore boreholes, three opencast prospecting boreholes and three collieries. Vitrinite-reflectance measurements consistently suggest that the seams have an anomalously lower degree of coalification than would be expected from their position in the geological succession. Higher volatile-matter yields from "whole-coal" samples of the Plessey seam, compared with the volatile-matter contents of coals immediately above and below in the succession, confirm this view. The suppressed vitrinite reflectances of the Plessey coals cannot be attributed to particular geothermal conditions and overlying seam lithologies, as other reduced reflectances of certain seams in the Durham coalfield have been in the past. Neither is the suppression caused by the now familiar close association that sometimes occurs between vitrinites and liptinitic macerals. The suppression and the raised volatile-matter yields are more likely to be caused by the original conditions under which the Plessey coals were deposited. The behaviour of the Plessey seam parallels that of the Katharina seam of the Ruhr coalfield, which, at the time of its deposition, was marine-influenced. Although no direct evidence exists that the Plessey seam was similarily affected, the close association of leaves of the Plessey seam in several boreholes with the Harvey Marine Band suggests that marine, near-marine or calcium-rich conditions, possibly aided by a high liptinite content in the marine band, are the most likely explanation for the production of the perhydrous coals. Bitumen exudates would have been generated early during the coalification process, so causing the aberrant properties and unexpectedly strong coking behaviour of the Plessey coals. (C) 2000 Elsevier Science Ltd. All rights reserved.