There are growing markets in developing countries, for instant WMP with the additional specification of being stable in coffee. Coffee is considered a harsh environment for reconstitution of milk powders owing to high temperatures (~80°C) and low pH (~4.8) which may cause protein precipitation or feathering. The objective of this study was to explore a range of parameters in order to identify some key factors that contribute to instability. Initially, the physicochemical conditions in the coffee test were scrutinised. Later, emphasis was shifted to the effects of individual process steps during pilot plant manufacture of instant WMP. Coffee-stable commercial samples were more resistant to feathering and sedimentation under conditions of low pH and high temperatures. A model test was developed to simulate the acidic effect of coffee, and to examine the sensitivity of WMP over a wider pH range. The resulting sedimentation/pH profiles were distinguishable for coffee-stable and coffeeunstable samples. Furthermore, the model test proved to be more exacting than the coffee itself. Subsequent investigations confirmed that monovalent cations such as K+ counteract the negative effects of low pH. Water hardness and powder characteristics such as bulk density, particle size and interstitial air volume influenced coffee-stability. The low sediment volumes that resulted from prior reconstitution before addition to coffee; increased mechanical activity and addition of surface active agents suggest that the observed instability may arise from the physical behaviour of the powder itself. In an investigation of the process parameters affecting coffee-stability, preheating and agglomeration conditions were critical to product physico-chemical characteristics and functionality. Fat composition of the milk influenced coffee stability more so than protein. Storage resulted in a disimprovement in coffee-stability. An alternative production method by recombination of cream into skim milk concentrate was comparable with the conventional process in terms of powder characteristics and coffee-stability as was also the case for butter-oil fat-filled powders.