Abstract Five occurrences of carbonaceous deposits formed in continuous coal or coal tar hydrogenation reactors were examined by optical microscopy with a view to determining the causes of deposit formation. Three of the deposits were formed in helically coiled reactor tubes and the other two in an open tubular reactor operated in the cocurrent upflow mode. It is apparent from the results obtained that the mode of deposition depends, to a large extent, on the nature of the vehicle oil used. In the two cases during which tar and recycle oil were utilized, deposition of carbonaceous materials was gradual. The nature of the deposits indicated that agitation within the reactor was insufficient to prevent settling of mineral matter, catalyst particles and mesophase. The major cause of reactor blockage for the three cases in which tetralin was used as vehicle oil was excessively high reaction temperature.