Apparatus and methods to monitor contamination levels in a formation fluid are disclosed. An example method involves obtaining first property data indicative of a first fluid property of a formation fluid and second property data indicative of a second fluid property of the formation fluid. A correlation between the first and second property data is generated and third data is fitted to the correlation. A fitting parameter is determined based on the third data indicative of an amount of change of the first property data relative to an amount of change of the second property data.