The authors deal with the design of high-rate, full-diversity, low-maximum likelihood (ML) decoding complexity space-time block codes (STBCs) with code rates of 2 and 1.5 complex symbols per channel use for multiple-input multiple output (MIMO) systems employing three and four transmit antennas. The authors fill the empty slots of the existing STBCs from coordinate interleaved orthogonal designs (CIODs) in their transmission matrices by additional symbols and use the conditional ML decoding technique, which significantly reduces the ML decoding complexity of non-orthogonal STBCs while ensuring full-diversity and high coding gain. First, two new schemes with code rates of 2 and 1.5 are proposed for MIMO systems with four transmit antennas. The authors show that our low-complexity rate-2 STBC outperforms the corresponding best STBC recently proposed by Biglieri et al. (2008) for quadrature phase shift keying (QPSK), due to its superior coding gain while our rate-1.5 STBC outperforms the full-diversity quasi-orthogonal STBC (QOSTBC). Then, two STBCs with code rates of 2 and 1.5 are proposed for three transmit antennas, which are shown to outperform the corresponding full-diversity QOSTBC. The authors prove by an information-theoretic analysis that the capacities of new rate-2 STBCs for three and four transmit antennas are much closer to the actual MIMO channel capacity than the capacities of classical OSTBCs and CIODs.