Abstract. The carbon dioxide (CO₂)-carbonate chemistry of sea-ice melt and co-located, contemporaneous seawater has rarely been studied in sea-ice-covered oceans. Here, we describe the CO₂–carbonate chemistry of sea-ice melt (both above sea-ice as "melt ponds" and below sea-ice as "interface waters") and mixed-layer properties in the western Arctic Ocean in the early summer of 2010 and 2011. At 19 stations, the salinity (∼0.5 to <6.5), dissolved inorganic carbon (DIC; ∼20 to <550 μmol kg⁻¹) and total alkalinity (TA; ∼30 to <500 μmol kg⁻¹) of above-ice melt pond water was low compared to the co-located underlying mixed layer. The partial pressure of CO₂ (pCO₂) in these melt ponds was highly variable (∼<10 to >1500 μatm) with the majority of melt ponds acting as potentially strong sources of CO₂ to the atmosphere. The pH of melt pond waters was also highly variable ranging from mildly acidic (6.1 to 7) to slightly more alkaline than underlying seawater (>8.2 to 10.8). All of the observed melt ponds had very low (<0.1) saturation states (Ω) for calcium carbonate (CaCO₃) minerals such as aragonite (Ω_aragonite). Our data suggest that sea-ice generated alkaline or acidic type melt pond water. This melt water chemistry dictates whether the ponds are sources of CO₂ to the atmosphere or CO₂ sinks. Below-ice interface water CO₂–carbonate chemistry data also indicated substantial generation of alkalinity, presumably owing to dissolution of CaCO₃ in sea-ice. The interface waters generally had lower pCO₂ and higher pH/Ω_aragonite than the co-located mixed layer beneath. Sea-ice melt thus contributed to the suppression of mixed-layer pCO₂, thereby enhancing the surface ocean's capacity to uptake CO₂ from the atmosphere. Our observations contribute to growing evidence that sea-ice CO₂–carbonate chemistry is highly variable and its contribution to the complex factors that influence the balance of CO₂ sinks and sources (and thereby ocean acidification) is difficult to predict in an era of rapid warming and sea-ice loss in the Arctic Ocean.