DNA, with its remarkable density and long-term stability, is an appealing potential next generation data storage medium, most notably for long-term archiving. Megabyte scale DNA based storage was first reported in 2012. The Shannon information capacity of DNA was recently demonstrated, using fountain codes, to be ~1.57 bit per synthesized position.However, synthesis and sequencing technologies process multiple nominally identical molecules in parallel, leading to significant information redundancies. We introduce composite DNA alphabets, using mixed DNA base types, to leverage this redundancy, enabling higher density. We develop encoding and decoding for composite DNA based storage, including error correction. Using current DNA synthesis technologies, we code 6.4Megabyte data into composite DNA, achieving ~25% increase in capacity as compared to literature. We further demonstrate, on smaller scales, how flexible synthesis leads to 2.7 fold increased capacity per synthesized position. Composite DNA can thus reduce costs for DNA based storage and can also serve in other applications.DNA based data storage systems are particularly appealing due to the high information capacityof DNA compared to current state of the art storage media. Storing digital information on DNA involves encoding the information into a sequence over the DNA alphabet (i.e. "A", "C", "G" and "T"), producing synthetic DNA molecules with the desired sequence, and storing the synthetic