For 40 years MAGIC has been used to simulate the acidification of soils and waters due to acid deposition. The original model has now been updated and re-implemented in the C++ Mobius platform and is available as open source. New features include multipoint calibration, forest growth, and a soil carbon module. The Mobius platform facilitates automatic optimisation of calibrated parameters and multiple-calibrations using Monte-Carlo routines. The usefulness of MAGIC is demonstrated by application to the 50-year data series for deposition and runoff at Birkenes, a small, calibrated catchment in southern Norway. Acid deposition has declined dramatically at Birkenes since the peak in the 1970s. Sulphate is 90% lower. Stream water has recovered strongly. Decreased concentrations of sulphate have led to increased acid neutralising capacity, pH and reduced concentrations of toxic aluminium. These changes are well-simulated by MAGIC. The new features added as part of Mobius improve the simulations. The MAGIC-Mobius modelling tool is now available for applications to scenarios of land-use and climate change.