Continuous renal replacement therapies (CRRTs) are increasingly used in order to maintain normal or near-normal acid-base balance in intensive care unit (ICU) patients. Acid-base balance is greatly influenced by the type of dialysis employed and by the administration route of replacement fluids. In continuous veno-venous hemofiltration, buffer balance depends on losses with ultrafiltrate and gain with replacement fluid, while in techniques such as continuous veno-venous hemodiafiltration, clinicians should balance the role of the dialysate. The type of buffer greatly influences not only acid-base correction, but also clinical outcome. Lactate or bicarbonate fluids are currently used, but recent studies suggest that bicarbonate-buffered replacement fluids can improve acid-base status and reduce cardiovascular events better than lactate fluids. The buffer concentration should exert a buffer load that may compensate for deficits, for losses in the buffer process, and for extracorporeal losses and should therefore usually be supraphysiological. However, the dialysate buffer or electrolyte concentration need always to be balanced with that of the replacement fluids employed. Both fluids should contain electrolytes in concentrations aiming for a physiologic level and taking into account preexisting deficits or excess and all input and losses. Clinicians should be aware that in CRRTs the quality control for sterility, physical properties, individualized prescription and balance control are vitally important.