Method of dissolving/mixing of a concentrate in/with a fluid in a multi-chamber bag and a method for the production of a medical fluid, in particular a dialysis fluid, in a multi-chamber bag. Moreover, the multi-chamber bag itself is disclosed. In all embodiments, at least two different concentrates can be included separately in powder form, liquid form or semi-liquid slurry form for dissolution in a fluid in the multi-chamber bag. In addition, the use of the multi-chamber bag in haemodialysis or peritoneal dialysis or a haemodialysis or peritoneal dialysis device, in particular as a container for a dialysis fluid in a haemodialysis or peritoneal dialysis device, is described.