Octahydridoborate, i.e. [B3H8](-) containing compounds, have recently attracted interest for hydrogen storage. In the present study, the structural, hydrogen storage, and ion conductivity properties of KB3H8 have been systematically investigated. Two distinct polymorphic transitions are identified for KB3H8 from a monoclinic (alpha) to an orthorhombic (alpha ') structure at 15 degrees C via a second-order transition and eventually to a cubic (beta) structure at 30 degrees C by a first-order transition. The beta-polymorph of KB3H8 displays a high degree of disorder of the [B3H8](-) anion, which facilitates increased cation mobility, reaching a K+ conductivity of similar to 10(-7) S cm(-1) above 100 degrees C. beta-KB3H8 starts to release hydrogen at similar to 160 degrees C, simultaneously with the release of B5H9 and trace amounts of B2H6. KBH4 and K-3(BH4)(B12H12) are identified as crystalline decomposition products above 200 degrees C, and the formation of a KBH4 deficient structure of K3-x(BH4)(1-x)(B12H12) is observed at elevated temperature. The hydrogen-uptake properties of a KB3H8-2KH composite have been examined under 380 bar H-2, resulting in the formation of KBH4 at T >= 150 degrees C along with higher metal hydridoborates, i.e. K2B9H9, K2B10H10, and K2B12H12.