There are two views on the mechanism of generation by some coals of excessive coking pressure and on laboratory testing; methods for coking coals. According to one of them, the excessive pressure is generated in a coal thermoplastic layer if its viscosity is too high. Another view is that some properties of a semicoke layer are responsible for generating high pressure. The two layers, i.e., semicoke and plastic layers, follow each other when they move from coke oven wall to the oven center plane during the coking process, The aim of our work was to seek an answer to whether thermoplastic properties of coals and volume contraction (or expansion) observed for their semicoke layers are interrelated. The following characteristics were measured for a set of 42 coals : (i) Gieseler thermoplastic properties determined in two independent laboratories using two different standard methods and (ii) contraction values of carbonized coals by the Koppers-INCAR test. In general, rather diffused or no relationships were found between the contraction on one hand and the thermoplastic properties on the other, It is concluded that the above two views might be synthesized on the condition that causative Links could be found between more specific characteristics of thermal decomposition during coal plastic state and contraction of semicoke layer, This issue is a subject of part 2 of the paper.