The pyrolysis of Stuart oil shale in the presence/absence of fully combusted shale (shale ash) in superheated steam or nitrogen was conducted in the range 450 to 600ยฐC in a 48 mm diameter semi-continuous fluidized-bed reactor. The detrimental effect of shale ash on oil yields in a recycled shale retorting system was confirmed. The coking of product oil vapour on shale ash led to oil losses that increased with increase in the ash-to-shale ratio. Reduction in surface area of the shale ash by high temperature treatment lowered oil coking losses. However, while pyrolysis in steam produced more oil than pyrolysis in nitrogen, the type of sweep gas (steam versus nitrogen) did not affect the rate of oil coking on shale ash. Certain types of clay minerals in the shale and changes in their structure during the course of pyrolysis/combustion were postulated as having a significant influence on the extent of the oil coking. Oil coking losses on shale ash, which were largely independent of temperature in the range 450 to 525ยฐC increased significantly with temperature above 525ยฐC. Except for methane and hydrogen, yields of gaseous products and the atomic H/C ratio and sulphur content of the oils were unchanged by the coking reactions.