To model intraday stock price movements we propose a class of marked doubly stochastic Poisson processes, whose intensity process can be interpreted in terms of the effect of information release on market activity. Assuming a partial information setting in which market agents are restricted to observe only the price process, a filtering algorithm is applied to compute, by Monte Carlo approximation, contingent claim prices, when the dynamics of the price process is given under a martingale measure. In particular, conditions for the existence of the minimal martingale measure Q are derived, and properties of the model under Q are studied.Keywords: Minimal martingale measure; news arrival; marked point process; nonlinear filtering; reversible jump Markov chain Monte Carlo; ultra-high frequency data. † Corresponding author. 1250018-1 Int. J. Theor. Appl. Finan. 2012.15. Downloaded from www.worldscientific.com by THE UNIVERSITY OF WESTERN ONTARIO on 04/12/15. For personal use only. 1250018-2 Int. J. Theor. Appl. Finan. 2012.15. Downloaded from www.worldscientific.com by THE UNIVERSITY OF WESTERN ONTARIO on 04/12/15. For personal use only. Monte Carlo Derivative Pricing in a Class of Marked DSPPtrajectory of the price process in any bounded time interval is characterized by a finite (although random) number of changes.An interesting feature of the framework proposed is that it can be interpreted to account for the link between the information release and the changes in price volatility and trading activity, whose existence has been many times suggested in the economic literature (see, among others, Engle and Ng [9] and Kalev et al. [17]). In our model, this link is embodied by the intensity process δ governing the speed of price changes. In particular, if δ is a shot noise process, its sudden increases can be interpreted as perturbations in market activity caused by pieces of news reaching the market, being the size of each increase due to the importance and unexpectedness of the news, and its consequent exponential decays can be interpreted as progressive normalizations due to the absorption of the effect of the news by the market.As far as the problem of pricing a contingent claim is concerned, a basic result of mathematical finance states that for a stochastic process S, representing the discounted stock price, the existence of an equivalent martingale measure, that is, of a measure equivalent to the "natural" probability P, such that S is a local martingale, is essentially equivalent to the absence of arbitrage opportunities (see, for example, Harrison and Kreps [15], Delbaen and Schachermayer [7]). If the price of the risky asset follows a marked point process, the market model is in general incomplete and it can be shown that there exist more then one of such equivalent measures. Thus, the problem of pricing a contingent claim, under the no arbitrage assumption, is reduced to taking expected values under the "right" measure among all existing equivalent martingale measures. One possibility is to choose the so cal...