The handling of powders in industry has been laden with problems caused by caking during storage and handling, whereby free flowing materials are transformed into lumps and agglomerates. It is of great importance for the manufacturers of bulk solid products to know whether and under what conditions the products are prone to cake. The objective of the work in this paper was to visually investigate the influence of relative humidity (RH) and humidity cycling on the caking behavior of powder particles in contact and to interpret the caking mechanism. A light microscope equipped with a digital camera was used to visually observe the caking of two particles in contact over time by taking a series of photographs. The following four food powders were studied: maltodextrin DE 21, sodium chloride (NaCl), skim milk powder (SMP), and a baby food powder. The maltodextrin and the baby food powders behaved similarly with sinter bridge formation being the caking mechanism. The NaCl and SMP behaved similarly with the caking mechanism being due to water bridge formation and solid bridge formation during low humidity exposure when the water vaporized leaving behind the dissolved solids in the bridge.