The dissolution of YBa2Cu306.8 in acidic aqueous solutions was studied at 298 K as a function of pH, nature and concentration of present anions and time, t. In the range 2.0 _ pH ___ 4.0, dissolution is congruent and proceeds to completion; dissolution profiles are deceleratory and comply with a contracting volume rate law up to t0.5. The rate of dissolution, which is limited by slow surface reactions, decreases with increasing pH; the kinetic order on proton concentration is a fractional and variable number. At higher pH values, dissolution is arrested at intermediate conversion values. In these cases, congruent dissolution is followed by the precipitation of less soluble solid phases which form a passive layer. The nature of this passive layer depends on pH and on the nature and concentration of present anions, which also define the extent of conversion attained at passivation. The dissolution behaviour of YBa2Cu3068 is described in terms of two consecutive surface reactions: fast equilibrated protonation of surface metal ions is followed by the slow release of cations. Barium surface ions are identified as the more reactive sites. The effect of the fast initial leaching of barium ions on the reactivity of yttrium and copper sites is discussed. The influence of the formal Cu(lll) oxidation state is also stressed.