I would like to know the price of Exxon in December 1990 and the
initial price of exxon/mobil when it merged in 1999.

Hi! Thanks for the question.

The price of Exxon for the different days in December of 1990 can be
found on the following link from Yahoo Finance.

Exxon Historical Prices: December 1990
http://finance.yahoo.com/q/hp?s=XOM&a=11&b=1&c=1990&d=11&e=31&f=1990&g=d


The merger of Exxon and Mobil was completed on November 30, 1999 and
was announced the next day. The closing prices on these dates were the
following:

30-Nov-99: 79.31
1-Dec-99:  82.50
Source: http://finance.yahoo.com/q/hp?s=XOM&a=10&b=30&c=1999&d=11&e=1&f=1999&g=d


Search strategy used:  
Searched for the term: exxon mobil merger
Searched for historical prices on Yahoo Finance: http://finance.yahoo.com/


I hope these links would help you in your research. Before rating this
answer, please ask for a clarification if you have a question or if
you would need further information.
                                                          
                              
Regards,                              
Easterangel-ga                              
Google Answers Researcher