How effective is advertising in duopoly markets?

A simple Ising spin model which can describe the mechanism of advertising in a duopoly market is proposed. In contrast to other agent- based models, the influence does not flow inward from the surrounding neighbors to the center site, but spreads outward from the center to the neighbors. The model thus describes the spread of opinions among customers. It is shown via standard Monte Carlo simulations that very simple rules and inclusion of an external field - an advertising campaign - lead to phase transitions, ie. extreme and fast changes in market share.