The latest conservation of wall paintings carried out in the chapel of Holy Trinity in the castle in Lublin was the th ird one in succession. The first conservation works combined with a discovery of polychromy were made in 1917—1918 and in 1921—1923 and then again after nearly forty years (1956/57/59) by a team of conservators associated with a state-owned enterprise of monuments conservation workshops (Warsaw branch). The latest conservation works covering studies, experimental and technical operations lasted from 1972 to 1979; in 1976—1979 a complete conservation of paintings in the presbytery was carried out (including a removal of the saltings, putting of fresh p u tty and lime patches, injections, artistic and aesthetic solutions). Conservation works in the chapel brought to light at least five different technical measures. The first and basis technical intervetnion was the strengthening of the internal structure of the walls in the whole chapel (the nave and presbytery) in which there were various non-identified oriffices and deep cracks as well as open or closed caverns re sulting from faulty technical solutions (a Polish weft with an outer facing made from bricks filled with ru b ble and with a lime-cum-sand mortar). The elimination of the said oriffices and damages was done by means of flushes of liquid mortar having hydraulic properties, prepared from Portland cement, lime and sand and a significant part of water. In those places of th e wall where liquid mortar with hydraulic properties was used, fresh patches and putty were made only after a full twenty-eight-day setting of the flush in order to avoid a possible diffusion of parts of cement to outer layers (carrying a painting layer, painting reconstructions and retouches). The second te chnical operation was the wedging (in the vaulting of the presbytery) of groins on joints with the eastern keystone. The joints produced triangles (due to a p a rtial slipping of the groins), in which the only points of bearing (i.e. of groins against th e keystone) were th e ir upper parts. To wedge, oak keys were applied; they were set in from the bottom and covered with proper putty. The th ird technical intervention was a removal of old putty and lime-cum-gypsum and cement patches which, as it is well-known, absorb moisture to a high extent and, as a result, bring about changes in the volume or surface changes in colour and value (painting reconstructions, retouches). Fresh patches and p u tty were made from lime-muc-sand mortar and lime mortar with a fine-grained lime filler. The fourth technical problem solved simultaneously with the putting of putty and lime patches were injections of pocketed parts of the plaster (protruding off the wall). Most of the injections were prepared from lime caseins with a 10 per cent addition of vinyl polyacetate in wate r dispersion made on the vaulting (about 75 per cent). The fifth technical operation was a removal of the salt from the surface of paintings, which in many places were poorly readable. Determinations of the samples taken from different places of the chapel (the nave and presbytery) have shown th a t these were water-soluble salts and th a t they are found both in plasters and in bricks. After a number of tests made in the whole chapel (employing the wet method by means of compresses and the dry one, mechanically) the salts found in the presbyte ry were removed by the dry technique, mainly with drafting gum.