According to the four-factor model of parental alienation, in order for alienation to be present there must be: (1) a prior positive relationship between the child and the now rejected parent; (2) absence of maltreatment by the rejected parent; (3) use of alienating behaviours by the favoured parent; and (4) presence of behavioural manifestations of alienation in the child. The purpose of the current study was to determine the reliability and validity of the four-factor model as a model of parental alienation. The study tested the reliability and validity of the four-factor model by having mental health professionals code vignettes representing a combination of presence and absence of the factors. Reliability was quite high across the vignettes, coders and factors. There was agreement that when all four factors are present the case is alienation and when one or no factor are present it is not alienation. These data support the four-factor model and suggest avenues for continuing to study the interplay among the factors deemed relevant by mental health professionals in the field of children’s relationships with their divorced parents.