In a duplex chat conversational trading system, trader terminals monitor for new incoming messages from the time a trader starts to input a new message to the time it is sent. If an incoming message is detected, sending of the message is inhibited and the trader is alerted to the new incoming message whereupon he can confirm his message, modify it or resend it. Messages are exchanged via a chat server which assigns a unique reference number to each message. Where a message received at the server is sent in reply to an earlier message it carries the reference of the earlier message. The server compares this reference to the most recent reference it has logged and notifies the parties of a crossover if the references are not the same.