typically sugar, which is known to form acidic bacteria harmful to enamel.
More seriously though is cocacola and pepsi cola which contain phosphoric acid. Soft drinks can contain different substances that harm your teeth. Sugars and acids play a significant role in tooth decay, and consequently in enamel destruction (the stuff a tooth is made off). Even fruit juices can cause harm when acids dissolve tooth enamel. Experts are suggesting that this acids can make tooth enamel soft and using a toothbrush straight after drinking can scratch the surface of the teeth and wear them down.