Martin Retro strings are made of an alloy known as monel. Monel alloy is a combination of copper and nickel with small amounts of iron, manganese, carbon and other materials which were used for Acoustic Guitars before Bronze became popular in the 1970s. This gives the string a very long lasting life due to the fact that it naturally repels oxidation. This string is specially designed to mellow out quickly so you don't get that harsh toppy sound that phosphor bronze provides.
Martin Retro strings bring out the woody vintage tone of your acoustic guitar. They deliver rich mid and low tones for rhythm parts as well as clear and clean notes for lead parts. The monel alloy makes the string feel a little more responsive and therefore allows you to be much more expressive. The range of gauges make these strings suitable for range of genres.