Two new reagents, nitrosobenzene (PhNO) and p-nitro-nitrosobenzene (p-NO2-PhNO), have been studied for the analysis of ascorbic acid (AA) by its hypochromic effect on the reagents. The partial multifactorial design of experiments coupled with analysis of parallelism of calibration lines was used to optimize the experimental conditions and evaluate the parameters which affect the sensitivity and accuracy. Two levels at which variable parameters are to be examined should be chosen carefully if a single-step examination is expected to be sufficient. If any parameter roughly influences the system it results not only in severe rotational and translational effects, but also in false results and low selectivity of the approach. In such a case a second step of examination is necessary. The optimization procedures indicated that both PhNO and p-NO2-PhNO are advantageous analytical reagents for the UV-spectrometric assay of AA at pH 3 when measuring the decrease of the absorbance of the reagent peak as analytical signal after 10 and 1 min, resp. After proper modelling both systems were sufficiently robust to be successfully applied for accurate, selective, sensitive, precise and simple analyses of AA even in pharmaceutical preparations with acetylsalicylic acid. Respective analytical performances characterizing AA-PhNO and AA-p-NO2-PhNO systems were: linearity range 2-100 and 2-70 micromol L(-1), CS 0.0094 and 0.0115 L micromol(-1), accuracy 101.5 and 98.0%, LOQ 1.3-2.1 and 3.1-3.6 micromol L(-1) and RSD 1.3-3.5 and 1.9-3.8%.