Reading comprehension (RC) is essential for academic success in all school subjects. Data indicate that several Senior Secondary School (SSS) students in Ogun State have deficiencies in English Reading Comprehension (ERC). The situation somewhat explains their inadequate performance in English language during public examinations. This research examined text quality characteristics (Text Readability-TR and Lexical Complexity-LC) as determinants of learning outcomes (performance) in ERC among public senior secondary students (SSSt) in Ogun State, Nigeria. The simple random sampling method was used to choose 2,500 SSII pupils, with 50 from each school. A purposive technique was used to select from one hundred teachers (two per school) teaching English in SSII. The Flesch readability formula was employed to assess the fluency level of the study's comprehension passage. The ERC Achievement Test (r=0.87), text readability (r=0.87), and LC (r=0.84) were additional instruments. Multiple regression (MR) at the 0.05 level of significance, descriptive statistics, and Pearson product-moment correlation (PPMC) were used to analyse the quantitative data. TR (x̄=3.11) was high, at the threshold of 2.50. LC was high (x̄=2.72) at the threshold of 2.50. TR (r = 0.23) and LC (r = 0.27) showed a substantial correlation with success in ERC. Two variables of LC (β = -.625) had significant contributions; TR (β = .090) had no significant contributions. Therefore, it can be concluded that only LC, among the independent factors, contributed individually to students' achievement (SA) in RC. Text complexity influenced SA in ERC among the selected SSS in Ogun State, Nigeria. Teachers should be mindful of these text quality factors in selecting RC passages.