A map is an involution (resp, anti-involution) if it is a self-inverse homomorphism (resp, antihomomorphism) of a field algebra. The main purpose of this paper is to show how split semi-quaternions can be used to express half-turn planar rotations in 3-dimensional Euclidean space R 3 and how they can be used to express hyperbolic-isoclinic rotations in 4-dimensional semi-Euclidean space R 3;1 . We present an involution and an anti-involution map using split semi-quaternions and give their geometric interpretations as half-turn planar rotations in R 3 . Also, we give the geometric interpretation of nonpure unit split semi-quaternions, which are in the form p = coshθ + sinhθi + 0j + 0k = coshθ + sinhθi, as hyperbolic-isoclinic rotations in R 3;1 .