A subcarrier allocation (SA) scheme was demonstrated to provide useful performance improvements for orthogonal frequency division multiplexing (OFDM) systems. Hence, in this letter, we extend its employment to OFDM with index modulation (OFDM-IM) systems. For the sake of improving the bit error rate (BER) performance, an optimal SA algorithm is first proposed by maximizing the average minimum Euclidean distance based on exhaustive search. Furthermore, for striking a better tradeoff between the complexity imposed and the BER performance attained, we propose a simplified SA algorithm by sorting the channel gains of the subcarriers. Simulation results show that with the aid of SA algorithms, significant performance gains can be achieved over conventional OFDM-IM systems.