Grain boundaries hold particular importance as they dictate the functional properties of polycrystalline materials, and this is reflected in the wealth of micromechanical research into their effects using primarily chemically homogeneous systems. Less understood are systems containing a degree of chemical heterogeneity, ubiquitous to applied materials, as found when impurity species segregate to grain boundaries. In approaching this topic, two distinct examples will be presented: (i) the strengthening effects of silver at Σ5 boundaries in copper studied by micropillar compression; (ii) the embrittling response of phosphorous at random high angle boundaries in tungsten studied through notched microcantilever beam bending experiments. Correlative atom probe tomography measurements linked to site-specific micromechanical testing allows for the observation that minute chemical segregation to grain boundaries can significantly affect key mechanical properties such as strength and toughness, while demonstrating that such questions can be studied using direct approaches.