The advantage of using K 2CO 3 or KCl as flux materials for material processing and crystal growth of RBa 2Cu 3O x ( R= Y, Gd, Dy, Pr, etc.) is reviewed. As far as their role in crystal growth is concerned, different possibilities are discussed. Large mosaic crystals with low misorientation can be obtained by using K 2CO 3 or KCl as additives during synthesis. Preparation temperatures in the range of 1270–1330 K are important as well as the slow cooling process. Under defined conditions, it is possible to obtain potassium-free products. With the exception of the non-superconducting Pr compound all compounds exhibit a distinct platelet morphology. In contrast to Gd, Dy and Pr derivatives, Y derivative shows a considerable nonstoichiometry and antisite disorder in the R and Ba sublattices. While oxygenation of the Y-123 crystals is achieved with ease, it is extremely difficult for rare earth derivatives. After oxygenation, the Y derivatives show a fairly sharp transition to the superconducting phase whereas the Gd and Dy derivatives show a broad transition with a width of ∼50 K. The difficulties in the oxygenation of rare earth derivatives are attributed to the absence of significant antisite disorder.