Ca diffusion was studied in c-axis and a-axis oriented YBCO films by Secondary Ion Mass Spectroscopy (SIMS). The films were grown by pulsed laser deposition (PLD). Ca(NO3)(2) solutions were deposited on the film surface and annealed over a large temperature range (300-870 degreesC) in air. The rate of Ca diffusion was measured to be the same in the c-axis and a-axis oriented films. The activation energy for Ca diffusion in YBCO was measured to be 67 +/- 10 kJ/mol. Fast diffusion along the grain boundaries was observed at temperatures below 700 degreesC with an activation energy of 46 +/- 12 kJ/mol. At 300 degreesC, the grain boundary diffusion coefficient is around 1.7 X 10(-14) cm(2) s(-1), which is 100 times larger than the corresponding bulk diffusion coefficient. (C) 2003 Elsevier B.V. All rights reserved.